A supplier cost always preceded by a minus sign on "vat period detail" form.

Hi Trevor,

It sounds like your purchase transaction may have the wrong VAT treatment.
Open the purchase transaction window and click More Options > VAT Treatment
> Purchase of Goods or Services from UK Supplier. Then save the
transaction and refresh the VAT Period Detail report.

Does that fix the problem?

Mark. Where is the purchase transaction window?
Trevor

Hi Trevor,

You can open the purchase transaction window by double-clicking on the
purchase in the Transactions list. It will be either a Purchase Invoice or
a Money Paid Out transaction with the Type set to 'General Payment'.