Maintain floor amount of 5.46 BSQ to prevent dust errors

When an offer is made using BSQ for trade fee, the BSQ amount
is burnt by doing a send-to-self.  However if the BSQ change
is below the bitcoin dust limit this causes an error.  We
fix this by maintaining a floor amount of 5.46 BSQ.

Fixes #4372

core/src/main/java/bisq/core/offer/OfferUtil.java

@@ -140,6 +140,10 @@ public static boolean isBsqForMakerFeeAvailable(BsqWalletService bsqWalletServic
         if (makerFee == null)
             return true;
 
+        Coin surplusFunds = availableBalance.subtract(makerFee);
+        if (Restrictions.isDust(surplusFunds)) {
+            return false; // we can't be left with dust
+        }
         return !availableBalance.subtract(makerFee).isNegative();
     }
 
@@ -171,6 +175,10 @@ public static boolean isBsqForTakerFeeAvailable(BsqWalletService bsqWalletServic
         if (takerFee == null)
             return true;
 
+        Coin surplusFunds = availableBalance.subtract(takerFee);
+        if (Restrictions.isDust(surplusFunds)) {
+            return false; // we can't be left with dust
+        }
         return !availableBalance.subtract(takerFee).isNegative();
     }
 

desktop/src/main/java/bisq/desktop/main/offer/MutableOfferDataModel.java

@@ -718,8 +718,14 @@ ReadOnlyObjectProperty<Coin> totalToPayAsCoinProperty() {
         return totalToPayAsCoin;
     }
 
-    Coin getBsqBalance() {
-        return bsqWalletService.getAvailableConfirmedBalance();
+    Coin getUsableBsqBalance() {
+        // we have to keep a minimum amount of BSQ == bitcoin dust limit
+        // otherwise there would be dust violations for change UTXOs
+        // essentially means the minimum usable balance of BSQ is 5.46
+        Coin usableBsqBalance = bsqWalletService.getAvailableConfirmedBalance().subtract(Restrictions.getMinNonDustOutput());
+        if (usableBsqBalance.isNegative())
+            usableBsqBalance = Coin.ZERO;
+        return usableBsqBalance;
     }
 
     public void setMarketPriceAvailable(boolean marketPriceAvailable) {

desktop/src/main/java/bisq/desktop/main/offer/MutableOfferView.java

@@ -373,9 +373,9 @@ private void showInsufficientBsqFundsForBtcFeePaymentPopup() {
         String message = null;
         if (makerFee != null) {
             message = Res.get("popup.warning.insufficientBsqFundsForBtcFeePayment",
-                    bsqFormatter.formatCoinWithCode(makerFee.subtract(model.getDataModel().getBsqBalance())));
+                    bsqFormatter.formatCoinWithCode(makerFee.subtract(model.getDataModel().getUsableBsqBalance())));
 
-        } else if (model.getDataModel().getBsqBalance().isZero())
+        } else if (model.getDataModel().getUsableBsqBalance().isZero())
             message = Res.get("popup.warning.noBsqFundsForBtcFeePayment");
 
         if (message != null)
@@ -1109,9 +1109,9 @@ private void showFeeOption() {
             String missingBsq = null;
             if (makerFee != null) {
                 missingBsq = Res.get("popup.warning.insufficientBsqFundsForBtcFeePayment",
-                        bsqFormatter.formatCoinWithCode(makerFee.subtract(model.getDataModel().getBsqBalance())));
+                        bsqFormatter.formatCoinWithCode(makerFee.subtract(model.getDataModel().getUsableBsqBalance())));
 
-            } else if (model.getDataModel().getBsqBalance().isZero()) {
+            } else if (model.getDataModel().getUsableBsqBalance().isZero()) {
                 missingBsq = Res.get("popup.warning.noBsqFundsForBtcFeePayment");
             }
 

desktop/src/main/java/bisq/desktop/main/offer/takeoffer/TakeOfferDataModel.java

@@ -639,8 +639,14 @@ public Coin getSellerSecurityDeposit() {
         return offer.getSellerSecurityDeposit();
     }
 
-    public Coin getBsqBalance() {
-        return bsqWalletService.getAvailableConfirmedBalance();
+    public Coin getUsableBsqBalance() {
+        // we have to keep a minimum amount of BSQ == bitcoin dust limit
+        // otherwise there would be dust violations for change UTXOs
+        // essentially means the minimum usable balance of BSQ is 5.46
+        Coin usableBsqBalance = bsqWalletService.getAvailableConfirmedBalance().subtract(Restrictions.getMinNonDustOutput());
+        if (usableBsqBalance.isNegative())
+            usableBsqBalance = Coin.ZERO;
+        return usableBsqBalance;
     }
 
     public boolean isHalCashAccount() {

desktop/src/main/java/bisq/desktop/main/offer/takeoffer/TakeOfferView.java

@@ -927,9 +927,9 @@ private void showFeeOption() {
             String missingBsq = null;
             if (takerFee != null) {
                 missingBsq = Res.get("popup.warning.insufficientBsqFundsForBtcFeePayment",
-                        bsqFormatter.formatCoinWithCode(takerFee.subtract(model.dataModel.getBsqBalance())));
+                        bsqFormatter.formatCoinWithCode(takerFee.subtract(model.dataModel.getUsableBsqBalance())));
 
-            } else if (model.dataModel.getBsqBalance().isZero()) {
+            } else if (model.dataModel.getUsableBsqBalance().isZero()) {
                 missingBsq = Res.get("popup.warning.noBsqFundsForBtcFeePayment");
             }
 
@@ -1225,9 +1225,9 @@ private void showInsufficientBsqFundsForBtcFeePaymentPopup() {
         String message = null;
         if (takerFee != null)
             message = Res.get("popup.warning.insufficientBsqFundsForBtcFeePayment",
-                    bsqFormatter.formatCoinWithCode(takerFee.subtract(model.dataModel.getBsqBalance())));
+                    bsqFormatter.formatCoinWithCode(takerFee.subtract(model.dataModel.getUsableBsqBalance())));
 
-        else if (model.dataModel.getBsqBalance().isZero())
+        else if (model.dataModel.getUsableBsqBalance().isZero())
             message = Res.get("popup.warning.noBsqFundsForBtcFeePayment");
 
         if (message != null)