Adjust invoice balance and apply payments to different invoices
I have a kinda unique request, my client has several invoices that are due, some of them from the past year and they are all billed to his non-profit organization. At the moment, he is willing to pay from his private business for some other work that I will do for him and he wants me to charge a little bit more on future invoices so that a portion of that payment will go to the non-profit bills that are due. For example, I will design an ad for his private business and I will charge a total of $100 on his business credit card. Out of that $100, $80 will be the actual cost for this job and the other $20 will go towards the non-profit organization bills that are due.
My question, is how can I split the payment to accurately mark the $100 invoice as paid and at the same time, put the $20 towards the non-profit bills. Or if it makes more sense, is there a way to add a discount or decrease the amount that is owed on invoices that are due? Will that be an issue as far as reconciliation goes cause some of those invoices are a year old. I appreciate any ideas and help.