There is no other way of offsetting the disbursement invoices other than as against the invoice you have now received.
Please follow the steps below:
Post a negative amount through the Firm Pre-Payment Screen to the exact value.
Allocate these figures off to contra the Firm Pre-Payment.
This will show an in and out on your Bank Reconciliation.
Pay the Disbursement invoices to the value of the Firm Pre-Payment.
In your Bank Reconciliation match these amounts to the original Firm Pre-payment.