Specific steps to accept and post pre-payment for multiple future invoices?

I have a customer from whom we accepted a massive pre-payment towards various proposals we have issued to them. I tried creating a deposit invoice with or without also converting it to an absolute discount for the customer.

Neither method does what I would expect.

I would expect that there is a way to post a credit amount to the customer’s account, then be able to post payment to invoices by debiting the credit balance.

Surely, there is a way to do this? Assuming so, would someone please point me in the right direction?

What if you put the data in the Capital field under the thirdparty?

I also vaguely remember a setting that asked me if I wanted to use individual accounts for each thirdparty, but I don’t remember where.