If the firm uses Firm level statement numbering (Utilities | Customization | Options tab), meaning each statement has a unique statement number, then it is easy to enter a payment for the correct client matter as long as you have a statement number.
To enter a payment based on statement number
Note: If your firm is on Client level statement numbering, we suggest running a Client Ledger Report (Reports | Management | Client Ledger Report) with a date range that includes the likely statement number. Select any other options, and/or narrow down the Client ID range, as desired. You can then preview the report and click the Search button to search for the statement number and determine the applicable client matter.