My customer wants to apply an invoice we incurred with them against an invoice they incurred with us.  How do I post that to show receipt of money and payment of invoice when no money changes hands?  eg.  We owe them $1600 and they owe us $1500 so they want to apply the amount they owe us to what we owe them and I am to cut a cheque for the difference.  I just can't figure out how to post that.    Help please.