Fix balance inconsistency

Sending the total promised value instead of a diff caused a behaviour where balance would be shown incorrectly.

Sending the diff fixes this.

Closes #1558

session/pingpong/exchange_message_tracker.go

@@ -242,7 +242,7 @@ func (emt *ExchangeMessageTracker) issueExchangeMessage(invoice crypto.Invoice)
 
 	defer emt.deps.Publisher.Publish(ExchangeMessageTopic, ExchangeMessageEventPayload{
 		Identity:       emt.deps.Identity,
-		AmountPromised: amountToPromise,
+		AmountPromised: diff,
 	})
 
 	// TODO: we'd probably want to check if we have enough balance here