I do not believe that this is possible using the Custom Reports functionality, because it is not possible to join multiple tables together - Custom Reports only query a single table at a time.
The way I handle similar situations is to set up Recurring Sales Invoices. If they have made an agreement with you to pay every week a certain amount, set up the recurring invoice to match the weekly payment.
- If they fall behind on their payments, you will clearly see that they owe you money in the Accounts receivable column of the Customers tab.
- At any given time, you can generate a Customer Statement which clearly shows a charge against their account every week, and the payments that they have made against their balance.
Note: Don’t forget to set an end date for the recurring invoice, if it applies! That way you won’t accidentally over-charge them.