When producing a bill if you have disbursement figures that need to be picked up the value can't just be amended on the bill these items need to be posted.
This can be done via various posting types depending how the payment is made:
You can create a Disbursement payment if this was paid out of office as a bacs/transfer.
If this is paid via a direct debit with lots of other matter invoices then it could be done as a Disbursement Journal.
Both of these posting types are in the Posting screen one is in Journals and the other in Payments.