When a credit note is raised, and saved as approved, the user will be prompted to apply the credit to invoices outstanding at the time.  Credit can only be applied to invoices for the same contact.

To use credit from a previously saved credit note, click on it in the tenants invoices area, or under the Credit Notes tab in Income and Expenses from the green menu, and select Allocate Credit from the menu at the top of the page.

A list of all invoices with balances due for the contact will show, enter the amount of the credit to allocate to each line as required and click Save Allocations.