There are "audit trail" templates you can acquire to track changes, but I don't think you need that for your database.
I'd define a related "payments" table and use a portal to this table on the subscriber layout to record each subscriber's payment. This gives you an ongoing list that can be set up like a small bookkeeping ledger where you enter each new payment on a new row in the portal.
I suggest looking up "Portal" in your book and in FileMaker help as your next step here. Then feel free to respond back with questions if you have any.