How to account for different payers on a single invoice????

We have a client that performs a lot of work for insurance adjusters.  They invoice the insurance adjuster for their services as time and materials.  Issue is that the invoice is actually paid by the insurance companies that the adjuster represents.  The client would like to know how to monitor how much each different insurance company has paid toward the invoice. 

It's almost like have an A/R sub ledger to the A/R sub ledger so to speak.  I was thinking of setting up a custom field on the invoice to reflect each insurance provider on a line item basis but was unsure of how to acknowledge their payment on the deposit.