Dear Saul John,
In Real Estate contracts you have to mention the number of Partners participated in the contract "Partners" tab ( As shown in Image 1), in your case Bill to customer, ship to customer..etc.
Then only system will recognise those partners for the postings, Then you can select the Financial customer (who is making the payment) for this contract from the dropdown list as shown below in image 2.
Here based on the requirement you can bill to more then one partner also by giving the % to pay by each partner, Any how in your case you are using single partner for payment so select that partner here and postings will be routed to that partner only.
Pls revert if need any more details.
Regards,
Rao