I have a horse boarding business and we invoice customers monthly for boarding.  I bought an item online for one of my customers for her convenience.  It was not related to the business.  She paid me back by adding the amount onto her check payment to me for her monthly boarding invoice.  So She was invoiced for $500 but paid me $550.  How do I edit the invoice to account for her reimbursement to me for the $50, without it being considered an income item.

I do not have a product or service item to account for a reimbursement.  How do I handle this to keep the accounting correct??

Thanks,