diff --git a/src/components/SendTokenDialog.vue b/src/components/SendTokenDialog.vue
index c90f9fa52..31afc22db 100644
--- a/src/components/SendTokenDialog.vue
+++ b/src/components/SendTokenDialog.vue
@@ -77,6 +77,14 @@
             style="max-width: 600px"
           >
             <ChooseMint />
+            <q-banner
+              v-if="paymentRequestMintWarning"
+              dense
+              rounded
+              class="bg-red-1 text-red-9 q-mt-sm"
+            >
+              {{ paymentRequestMintWarning }}
+            </q-banner>
           </div>
         </div>
 
@@ -395,6 +403,38 @@ export default defineComponent({
           this.isValidPubkey(this.sendData.p2pkPubkey)
       );
     },
+    // NUT-18's `m` field is mandatory today (cashu-ts `PaymentRequest.mints`
+    // is a plain `string[]`). This constraint helper wraps that so when the
+    // spec change at https://github.com/cashubtc/nuts/pull/381 lands we can
+    // also return `{ kind: "preferred", allowed }` without touching callers:
+    // the button disable + warning logic just keys off `kind`.
+    paymentRequestMintConstraint(): {
+      kind: "mandatory";
+      allowed: string[];
+    } | null {
+      const allowed =
+        this.sendData.paymentRequest?.mints?.filter((m): m is string => !!m) ??
+        [];
+      if (allowed.length === 0) return null;
+      return { kind: "mandatory", allowed };
+    },
+    selectedMintViolatesRequest(): boolean {
+      const c = this.paymentRequestMintConstraint;
+      if (!c) return false;
+      if (!this.activeMintUrl) return true;
+      return !c.allowed.includes(this.activeMintUrl);
+    },
+    paymentRequestMintWarning(): string {
+      // Only block (and warn) for mandatory constraints. Preferred-mint
+      // copy can be added later as a separate i18n key without reshaping
+      // this method.
+      const c = this.paymentRequestMintConstraint;
+      if (!c || c.kind !== "mandatory") return "";
+      if (!this.selectedMintViolatesRequest) return "";
+      return this.$t(
+        "SendTokenDialog.errors.mint_not_allowed_by_request"
+      ) as string;
+    },
     paymentRequestButtonDisabled(): boolean {
       if (!this.sendData.paymentRequest) {
         return true;
@@ -409,10 +449,24 @@ export default defineComponent({
       if (this.globalMutexLock) {
         return true;
       }
+      const c = this.paymentRequestMintConstraint;
+      if (c && c.kind === "mandatory" && this.selectedMintViolatesRequest) {
+        return true;
+      }
       return false;
     },
   },
   watch: {
+    activeMintUrl: function (newUrl, oldUrl) {
+      // When the user switches mint inside the Pay-PaymentRequest sheet, any
+      // proofs we already prepared belong to the previous mint (and embed
+      // that mint URL in the serialized token). Drop them so the next pay
+      // attempt rebuilds from the newly-selected mint.
+      if (!this.showSendTokens) return;
+      if (!this.sendData.paymentRequest) return;
+      if (!newUrl || !oldUrl || newUrl === oldUrl) return;
+      useSendTokensStore().invalidatePreparedPaymentRequestToken();
+    },
     showSendTokens: function (val) {
       if (val) {
         this.$nextTick(() => {
diff --git a/src/i18n/cs-CZ/index.ts b/src/i18n/cs-CZ/index.ts
index ef6aa2d26..3f3f64bf0 100644
--- a/src/i18n/cs-CZ/index.ts
+++ b/src/i18n/cs-CZ/index.ts
@@ -1050,6 +1050,8 @@ export default {
     errors: {
       amount_required: "Nejprve zadejte částku.",
       serialization_failed: "Nepodařilo se připravit ecash token.",
+      mint_not_allowed_by_request:
+        "Vybraný mint není tímto platebním požadavkem přijímán.",
     },
   },
 
diff --git a/src/i18n/en-US/index.ts b/src/i18n/en-US/index.ts
index d81129da5..e7cecaba9 100644
--- a/src/i18n/en-US/index.ts
+++ b/src/i18n/en-US/index.ts
@@ -1040,6 +1040,8 @@ export default {
     errors: {
       amount_required: "Enter an amount first.",
       serialization_failed: "Could not prepare ecash token.",
+      mint_not_allowed_by_request:
+        "The selected mint is not accepted by this payment request.",
     },
   },
   SendPaymentRequest: {
diff --git a/src/i18n/pt-BR/index.ts b/src/i18n/pt-BR/index.ts
index 265bfce50..2656a369b 100644
--- a/src/i18n/pt-BR/index.ts
+++ b/src/i18n/pt-BR/index.ts
@@ -1047,6 +1047,8 @@ export default {
     errors: {
       amount_required: "Insira um valor primeiro.",
       serialization_failed: "Não foi possível preparar o token ecash.",
+      mint_not_allowed_by_request:
+        "O mint selecionado não é aceito por esta solicitação de pagamento.",
     },
   },
   SendPaymentRequest: {
diff --git a/src/stores/proofs.ts b/src/stores/proofs.ts
index ff95e963f..dcbfa8c05 100644
--- a/src/stores/proofs.ts
+++ b/src/stores/proofs.ts
@@ -39,8 +39,13 @@ export const useProofsStore = defineStore("proofs", {
       },
     });
 
-    // Function to update activeProofs
-    const updateActiveProofs = async () => {
+    // Filter the live in-memory proofs ref instead of re-querying cashuDb.
+    // The liveQuery above keeps `proofs.value` in sync with the table, so
+    // doing this synchronously lets activeProofs update in the same tick
+    // as activeMintUrl/activeUnit — otherwise downstream getters like
+    // activeBalance briefly read the old mint's proofs and the UI flashes
+    // an "insufficient balance" warning before the async fetch resolves.
+    const updateActiveProofs = () => {
       const mintStore = useMintsStore();
       const currentMint = mintStore.mints.find(
         (m) => m.url === mintStore.activeMintUrl
@@ -58,15 +63,10 @@ export const useProofsStore = defineStore("proofs", {
         return;
       }
 
-      const keysetIds = unitKeysets.map((k) => k.id);
-      const activeProofs = await cashuDb.proofs
-        .where("id")
-        .anyOf(keysetIds)
-        .toArray()
-        .then((proofs) => {
-          return coerceWalletProofs(proofs).filter((p) => !p.reserved);
-        });
-      mintStore.activeProofs = activeProofs;
+      const keysetIds = new Set(unitKeysets.map((k) => k.id));
+      mintStore.activeProofs = proofs.value.filter(
+        (p) => keysetIds.has(p.id) && !p.reserved
+      );
     };
 
     return {
diff --git a/src/stores/sendTokensStore.ts b/src/stores/sendTokensStore.ts
index 9a0249e45..afd27d416 100644
--- a/src/stores/sendTokensStore.ts
+++ b/src/stores/sendTokensStore.ts
@@ -37,5 +37,18 @@ export const useSendTokensStore = defineStore("sendTokensStore", {
       this.sendData.paymentRequest = undefined;
       this.sendData.historyToken = undefined;
     },
+    // Drops any token/proofs that were prepared for the current PaymentRequest
+    // so the next pay attempt rebuilds them from the currently-active mint.
+    // No-op when no PR is active (regular send flows keep their tokensBase64
+    // until the dialog closes).
+    invalidatePreparedPaymentRequestToken(): boolean {
+      if (!this.sendData.paymentRequest) return false;
+      if (!this.sendData.tokensBase64) return false;
+      this.sendData.tokens = "";
+      this.sendData.tokensBase64 = "";
+      this.sendData.historyToken = undefined;
+      this.sendData.historyAmount = null;
+      return true;
+    },
   },
 });
diff --git a/test/vitest/__tests__/sendTokensStore.test.ts b/test/vitest/__tests__/sendTokensStore.test.ts
new file mode 100644
index 000000000..0c6867ef8
--- /dev/null
+++ b/test/vitest/__tests__/sendTokensStore.test.ts
@@ -0,0 +1,62 @@
+import { test, describe, expect, beforeEach } from "vitest";
+import { setActivePinia, createPinia } from "pinia";
+import { useSendTokensStore } from "src/stores/sendTokensStore";
+
+beforeEach(() => {
+  setActivePinia(createPinia());
+});
+
+describe("sendTokensStore.invalidatePreparedPaymentRequestToken", () => {
+  test("no-op when there is no active payment request", () => {
+    const store = useSendTokensStore();
+    store.sendData.tokensBase64 = "cashuB-xxxxxx";
+    store.sendData.tokens = "stale-proofs";
+    store.sendData.historyToken = { id: "h1" } as any;
+    store.sendData.historyAmount = -42;
+
+    const changed = store.invalidatePreparedPaymentRequestToken();
+
+    expect(changed).toBe(false);
+    // Regular send flows must keep their cached token until the dialog closes.
+    expect(store.sendData.tokensBase64).toBe("cashuB-xxxxxx");
+    expect(store.sendData.tokens).toBe("stale-proofs");
+    expect(store.sendData.historyToken).toEqual({ id: "h1" });
+    expect(store.sendData.historyAmount).toBe(-42);
+  });
+
+  test("no-op when there is no cached token to invalidate", () => {
+    const store = useSendTokensStore();
+    store.sendData.paymentRequest = { id: "pr1" } as any;
+
+    const changed = store.invalidatePreparedPaymentRequestToken();
+
+    expect(changed).toBe(false);
+    expect(store.sendData.paymentRequest).toEqual({ id: "pr1" });
+  });
+
+  test("clears cached PR token state so the next pay attempt rebuilds", () => {
+    const store = useSendTokensStore();
+    store.sendData.paymentRequest = {
+      id: "pr1",
+      mints: ["https://trusted.example"],
+    } as any;
+    store.sendData.tokensBase64 = "cashuB-from-wrong-mint";
+    store.sendData.tokens = "stale-proofs";
+    store.sendData.historyToken = { id: "h1" } as any;
+    store.sendData.historyAmount = -42;
+
+    const changed = store.invalidatePreparedPaymentRequestToken();
+
+    expect(changed).toBe(true);
+    expect(store.sendData.tokensBase64).toBe("");
+    expect(store.sendData.tokens).toBe("");
+    expect(store.sendData.historyToken).toBeUndefined();
+    expect(store.sendData.historyAmount).toBeNull();
+    // The payment request itself must stay intact – only the prepared
+    // proofs/token are invalidated.
+    expect(store.sendData.paymentRequest).toEqual({
+      id: "pr1",
+      mints: ["https://trusted.example"],
+    });
+  });
+});