I appreciate you returning to this thread and providing a screenshot, Dode. Allow me to clarify things so you can match transactions effectively in QuickBooks Online (QBO).

The error message you're encountering appears when you use the Resolve difference option to adjust fees. This situation doesn't apply because you're dealing with an open invoice. When you attempt to match both transactions, it leads to a partial payment scenario.

To have accurate records, you don't need to match the bank transaction. Instead, you can create a Receive payment to that invoice:

1. Go to the + New icon and select Receive Payment.
2. Select the Undeposited Funds to Deposit to section.
3. Select the invoice transaction, then put a checkmark on it.
   
   
   
   
4. Click Save and close.

QuickBooks will automatically create a payment transaction for the fees. Then, let's create a Bank deposit to account for the fees:

1. Go to the + New icon and select Bank Deposit.
2. Select the bank account associated with the downloaded bank transaction.
3. Select the payment transaction amounting to the £441.12 and £6.88.
4. Then enter a negative amount £6.88 in the Add funds to this deposit section.
   
   
   
   
5. Select an expense account you used to track fees.
6. Click Save and Close.

Once done, let's go back to the Bank Transactions tab and then match the deposit amount to the downloaded bank transaction:

1. Select the bank transaction.
2. Click the Deposit.
   
   
   
   
3. Select Match.

Moreover, you can generate the accounts receivable aging report in QuickBooks to gain valuable insights into your customers' outstanding balances. This report provides a comprehensive overview of which clients are falling behind on their payments and still owed which helps you prioritize follow-ups with customers who may need reminders or assistance.

If you still have questions about matching bank and existing transactions in QuickBooks, just let me know so we can provide prompt assistance. Have a good one.