Hi,
You could play around with the repayment structure. SAP delivers three: Final, Annuity and Partial. If you use partial, it'll let you select frequency and if you do not put a desired amount, it will calculate equal payments based on the frequency. I put some snapshots together. These options are great if you want the system to calculate it automatically for you. You could also just do a principal decrease cash flow for the date they want to make it but the user will have to enter the amount and the date they want to make the payment. You can enter principal decrease by clicking on the button next to the currency. Hopefully that gives you some direction.
-Anya