To model the partial repayment of a mortgage, use the Debt > Step Up /Step Down feature to step up the mortgage payment at a given event. If this is a one-off paydown, step the payment back down to regularly scheduled (or adjusted) levels in the following year of the plan.
To model a (one-off) partial repayment of a mortgage (not a payoff, which is even easier):
1. First add two events to the timeline. Go to the Time screen.
2. Drag and drop an event onto the timeline in the year these two windfalls/inflows are to be realised. Name it Mortgage Repayment, for example.
3. Place a second event in the following year of the plan. Name it Resume Regular Mortgage Payment, for example.
4. Go to the Debt screen.
5. Select the debt for the property in the Ledger.
6. Next, expand the Advanced Settings panel and click on Step Up/Step Down.
7. Select the first of the two events to schedule the overpayment. Enter the stepped up payment amount. Inflation rate can be left as is.
Another set of fields will display automatically on the Step Up / Step Down panel allowing you to enter an additional step.
8. Select the second of the two events and enter the regular mortgage payment amount, stepping it down to the usual amount in the year following the over payment.
9. Click Update to save these changes.
Check the details of the Let's See chart to verify that this is being modeled as expected.