When a payment exceeds the total amount due on an invoice, Teachworks automatically applies the matching amount to that invoice. The surplus amount is saved on the client's profile as an unallocated credit, which can be applied to future invoices or refunded.
If a client has any unallocated credits, a notification line will automatically appear on their future invoices directly below the "Totals" section. Click the link within this notification line to instantly draw from the available credit balance and apply it to the invoice.
You can read more about allocating credit to an invoice here: Allocating Credit or Unallocated Payments to an Invoice.