If you don't need to reverse the original bill and are happy to give a credit for the disbursement you can follow the below steps:
Select General Postings.
Post a minus of the disbursement type you are using BPO/ND/AD/PIO/PIN/PPO/PPN.
Bill the minus disbursement if required by doing an SIB posting in General Postings.
Transfer the credit of any monies back to client via a TOC posting if the original bill has already been paid.
These monies would need to be allocated against the disbursement minus bill you have just done.