If you need to bill the VAT element then doing a disbursement posting with just VAT on will not show when trying to bill.
A minus of the original with no VAT and a new posting with VAT will need to be created as below:
From the Main Menu go into the General Postings.
Select the transaction type of Disbursement eg AD/BPO.
Select NEW.
Put the matter in and all the details required with the VAT code used on the first posting with no VAT as a minus value.
Select the floppy disk icon to save.
Produce another posting this time as a positive and with a VAT value.
Select the floppy disk icon to save again.
Click POST at the bottom.