Show the Invoices where Payment was applied to under Transactions of a Customer Record

1.    Navigate to Customer> Sales> Transaction> Hit Customize View.

2.    Users will then be taken into a Saved Transaction Search. Rename it to your desired title, e.g. Payment Transaction Only.

3.    On the Criteria > Standard tab, add the following:
       > Type is Payment
       > Account is Accounts Receivable

4.    On the Results tab> Column tab> add Applied To Transaction  

5.    Hit Save.

6.    Navigate back to your customer record >Transactions tab > set Type=Payment and View=To the name you set on the saved search (e.g. Payment Transaction Only). The Invoice number will be shown on the Applied To Transaction column.