Payment for installment of the product

Hi

How can I manage Payment for installment of the products in CRM 365? could I use contract?

e.g. customer buy product , When the timing of each installment comes , sales person enter amount of that and check mark that month!

September  ✅$100   paid

October      ✅$100    paid

November  ✅ $100   paid

December     0        not paid

FYI: I try to add these field on contract form but problem is when contract status will be actives there is no way to modify fields, then sales person can't update these fields !

Any idea?

Thanks,