When you adjust a salesperson's Withholding Tax in OfficePartner, you may need to recalculate the WT against an existing invoice. To do this, follow these steps:
Open OfficePartner -> SalesPeople
Double click into the desired salesperson, and click on the Invoices tab.
Right click on the invoice item you wish to recalculate and click Reset Commission Details.
If an invoice item is Completed, you will need to Uncomplete / Click <UC>, before you can recalculate the commission. Should you do so, follow the default prompt options IE. Deduct commission from YTD totals? = Yes