Invoicing Details and Reflecting Payments for Each

I have a client that has 15 different entities. He wants his billings to be detailed on one invoice, showing the amounts that each entity owes and the total that the parent company owes.

When I receive payment for same, I would like to apply the proper amount to each of the entities, not to the parent company, so that I can produce a statement that details which of the 15 entities still owes and for which invoice.

Any suggestions?