Applying one deposit to another to move over the commitment balance

We have the following requirement,

1. Customer initially requests an invoice to purchase a pre-paid credit say £1,000 (More like purchasing an iTunes credit but requiring an invoice to pay for it)

2. So we create one deposit and then raise that as PDF and then customer pays against it so cash receipt applied against the Deposit takes the Receivables balance to zero

3. Then there are multiple purchases from the customer and hence invoices created to draw down the balance of the commitment - say now with 9 invoices worth of £900 in total

4. When the commitment balance reaches a minimum threshold (say £100) then customer receives a notification for purchasing additional pre-paid credit - say further £1,000