Once both people sign a shared agreement, its money terms lock, and the installment plan is part of those terms. That protects both of you: the schedule you signed is the schedule that counts.
What you can do
- Keep paying in parts anyway. You can record any payment at any time; the balance updates. See Partial payments.
- Start fresh. Agree on a new schedule, create a new agreement for the remaining balance with an installment plan, sign it, and then delete or settle the old one.
Tip: Next time, set up the installment plan right after creating the agreement, and sign after.