My idea began as a Pledges table and a Transactions table.
A Pledge record would be created for the individual's, and include the full amount, a start and end date, a frequency (monthly, quarterly, etc.). Then it could list the schedule to fulfill that pledge on time.
Then, a Transaction record would be created for the individuals actual donation towards the pledge, and this could update the Pledge record showing that the remaining balance is the [Total] - [Transaction Amount].
With the schedule, could it also be possible to be able to note when the balance is in arrears when a transaction hasn't been added in time to meet the set schedule.