Thanks for posting here, @Usman01,

At the moment, you can only link transactions with the same currency in QuickBooks Online. We're unable to match a GBP payment towards a EUR invoice.

Your case is a special situation and so it needs a few steps to do. First, we’ll first have to create a new dummy bank account in the home currency to facilitate the movement between 2 different foreign currencies.

1. Select the Gear icon, then select Chart of Accounts
2. Press New
3. Under Account Type column, select Cash at bank and in hand. Under Detail Type column, select Cash on hand. And under Name column, type in Dummy Bank.
   
4. Before saving and closing, make sure that it is in the home currency (which is GBP in this case)

We’ll first have to record payment of the foreign invoice. In the Receive Payment module, set the Deposit To field to Dummy Bank. Before saving the transaction, take note of the home currency (GBP) amount displayed at the bottom right.

Next, we’ll have to transfer this GBP amount from Dummy Bank to the EUR bank account.

1. Go to the + New icon and select Transfer.
2. Under Transfer Funds From column, select Dummy Bank from the drop-down arrow.
3. Now, in the Transfer Funds To, select EUR Bank from the drop-down arrow.
4. In the Currency column, select GBP from the drop-down arrow.
5. For the Transfer amount column, type in the amount.
6. Under Exchange rate, the exchange rate will automatically appear. But you can manually change the rate.

If you have any questions, let me know in the comment. I'll be right here to help you with the customer payments. Have a good one!