1. In Accounts Receivable > Records > Clients, open Client B (the client who should be paying for the item) and select their Statements tab.
2. Ensure that the appropriate number of rows are listed and that there aren't any extra rows for the other payer that would list "All Charges"
3. If there are, delete these rows and save and close the client

The invoice or charge should now no longer show under the incorrect client.