We can achieve this by creating two new candidate timesheets one as a positive amount without VAT and the other as a negative with VAT, with the resulting payment then for VAT only. Firstly create a timesheet via the Incoming Timesheet Images area by clicking the 'Add new timesheet' button:
Select the correct assignment, set the timesheet periods to suit the period of the adjustment and click the Show Complex Rates option to separate out the pay and bill lines on the timesheet:
Delete any surplus lines so that only the pay line remains and enter the unit amount as a negative to represent the correction you wish to make. Tick the Invoice without Timesheet Image box then validate the timesheet - a message will be display regarding the pay and bill amounts not being equal but this can be overridden:
Next create another timesheet in the same way, this time with a positive amount of units with the correct VAT rate alongside and validate this in the same way as shown above.
The two timesheets will be waiting in the Supplier Payment area - select them both and click to produce the remittance payment showing the VAT amount only: