Help me to be a responsible loan shark.

I'm selling a piece of property to someone with a seller-financed loan -- so I make the money from the power of compound interest instead of a bank. We've got lawyers handling the paperwork for the transaction. However, I've never been a loan shark before. Suggestions for financial software to manage the loan so I can enter payment amounts and dates and such and keep the magic of compound interest accurately compounding? I'm platform agnostic among OSX, WinXP, and various linuxes. I prefer something that's actual software instead of a service that I have to keep paying money for (Quicken and your expiring ability to download activity, I'm looking at *you* right now). I am okay with entering the payment and date data by hand, but I'd like to be able to export the records so I can share them with the buyers periodically. I find the idea of an excel spreadsheet kind of sketchy, mostly because I don't trust my own ability to correctly make the formulae.