I would think that Numbers could do that for you. A basic spreadsheet. There seem to be many templates for that kind of need. Just search for something like "spreadsheet to track loan payments". Most will be for Excel but Numbers can use those if the math is not too complicated. I'd look for one that tracks payments (Yes/No) in one column, of course, you might want to add a column for 'date of payment' (if not included in the template.
