These are my suggestions as to how to do this. There are more ways to do this, but for what you have described I think this method would give you what you're looking for.
Create a list of parent customers who you collect on behalf of. Create sub-customers under the appropriate parent customers; the names of people/companies who you are actually collecting the money from.
Create an invoice for the sub-customer (which is automatically listed under it's parent). Create a Product/Service called 'Deposits Payable' or whatever you want to call it. Make it a Service type item. Put in the Description you want to show on the invoices. Under Income Account, put in a Current Liability account called 'Deposits Payable' or whatever you want to call it. Save and close.
You have now created an invoice to the place you want to bill, but instead of going to an income account (because, as you said, it is not your sale), it is going to a liability account and will show up on your balance sheet as a liability, until such time as you reduce the liability by paying your parent customer. Yes, it is still a receivable as well, but it has reduced your overall receivables while increasing your liabilites. When you pay your parent customer, of course the opposite will occur; receivable is increased and liability is decreased.
Create a custom transaction report. Filter by the Deposits Payable account, and choose customer names, and total by customer. Memorize the report for future use. This report will let you know at all times how much is owing to your parent customer, but will also show the breakdown of any sub-customers underneath them.
