tapchannel: also check local buy quotes in bandwidth check

Because the traffic shaper is now also used for checking the bandwidth
when forwarding HTLCs to the final recipient, we also need to check our
local buy quotes for a quote with the given RFQ ID.

tapchannel/aux_traffic_shaper.go

@@ -213,9 +213,21 @@ func (s *AuxTrafficShaper) PaymentBandwidth(htlcBlob,
 	// up the accepted quote and determine the outgoing bandwidth in
 	// satoshis based on the local asset balance.
 	rfqID := htlc.RfqID.ValOpt().UnsafeFromSome()
-	acceptedQuotes := s.cfg.RfqManager.PeerAcceptedSellQuotes()
-	quote, ok := acceptedQuotes[rfqID.Scid()]
-	if !ok {
+	acceptedSellQuotes := s.cfg.RfqManager.PeerAcceptedSellQuotes()
+	acceptedBuyQuotes := s.cfg.RfqManager.LocalAcceptedBuyQuotes()
+
+	sellQuote, isSellQuote := acceptedSellQuotes[rfqID.Scid()]
+	buyQuote, isBuyQuote := acceptedBuyQuotes[rfqID.Scid()]
+
+	var rate rfqmsg.AssetRate
+	switch {
+	case isSellQuote:
+		rate = sellQuote.AssetRate
+
+	case isBuyQuote:
+		rate = buyQuote.AssetRate
+
+	default:
 		return 0, fmt.Errorf("no accepted quote found for RFQ ID "+
 			"%x (SCID %d)", rfqID[:], rfqID.Scid())
 	}
@@ -224,7 +236,7 @@ func (s *AuxTrafficShaper) PaymentBandwidth(htlcBlob,
 	// expressed in milli-satoshis.
 	localBalanceFp := rfqmath.NewBigIntFixedPoint(localBalance, 0)
 	availableBalanceMsat := rfqmath.UnitsToMilliSatoshi(
-		localBalanceFp, quote.AssetRate.Rate,
+		localBalanceFp, rate.Rate,
 	)
 
 	// At this point we have acquired what we need to express the asset