What I have is a purchase for say £100 for Microsoft Licenses. So I paid £100 to the company, but the exchange rate worked out that the actual bill was £97. So my invoice is £97, but I have actually paid £100 on the bank account, so I have £3 credit with the company.

Where do I put this £3 as I am not ofsetting the amount against any invoice as the invoice is £97.