Hi thereHow can I post a contra between a supplier who is also a customer if the customer pays in GBP and we pay him as a supplier in USD

Hello MarieL2, thanks for joining the Community

The usual way to record a contra payment would be to create a journal entry with the default Debtors and Creditors account selected on either line (and the appropriate Customer/Supplier in the NAME field). This journal can then be linked as payment on an invoice or bill. 

However, QuickBooks Online blocks journal entries that mix currencies for customers or vendors because these may lead to incorrect open balances or incorrect exchange gains or losses.

Our guide linked here provides a few options on what to do in this circumstance - we'd recommend working alongside your bookkeeper or accountant if you're less comfortable with accounting, to ensure that this is recorded correctly. 🙂