It would be great to be able to apply credits to a client's account that can then be used for upcoming invoices. Even better, having an option to choose between manual and automatic, i.e. an automatic credit would automatically apply itself to future invoice(s). If the next invoice is less than the credit amount, then the invoice would be marked as paid in full and the remaining credit balance would be applied to future invoice(s).

For reference, here's how Freshbooks does it: https://support.freshbooks.com/hc/en-us/articles/115011590827-How-do-credits-work/