rpcserver: refactor validateInvoiceAmount

We move the validation around in preparation for the next commit that
adds more logic. The diff will be smaller and focused on the important
change thanks to this move.

Author: GeorgeTsagk

rpcserver.go

@@ -7898,17 +7898,6 @@ func (r *rpcServer) AddInvoice(ctx context.Context,
 func validateInvoiceAmount(acceptedQuote *rfqrpc.PeerAcceptedBuyQuote,
 	requestAssetAmount uint64) (lnwire.MilliSatoshi, error) {
 
-	// If the invoice is for an asset unit amount smaller than the minimal
-	// transportable amount, we'll return an error, as it wouldn't be
-	// payable by the network.
-	if acceptedQuote.MinTransportableUnits > requestAssetAmount {
-		return 0, fmt.Errorf("cannot create invoice over %d asset "+
-			"units, as the minimal transportable amount is %d "+
-			"units with the current rate of %v units/BTC",
-			requestAssetAmount, acceptedQuote.MinTransportableUnits,
-			acceptedQuote.AskAssetRate)
-	}
-
 	// Now that we have the accepted quote, we know the amount in Satoshi
 	// that we need to pay. We can now update the invoice with this amount.
 	//
@@ -7929,6 +7918,17 @@ func validateInvoiceAmount(acceptedQuote *rfqrpc.PeerAcceptedBuyQuote,
 		assetAmount, *askAssetRate,
 	)
 
+	// If the invoice is for an asset unit amount smaller than the minimal
+	// transportable amount, we'll return an error, as it wouldn't be
+	// payable by the network.
+	if acceptedQuote.MinTransportableUnits > requestAssetAmount {
+		return 0, fmt.Errorf("cannot create invoice for %d asset "+
+			"units, as the minimal transportable amount is %d "+
+			"units with the current rate of %v units/BTC",
+			requestAssetAmount, acceptedQuote.MinTransportableUnits,
+			acceptedQuote.AskAssetRate)
+	}
+
 	return newInvoiceAmtMsat, nil
 }