No, it is not possible to make positive adjustment for card & bank payment methods. Only negative adjustments i.e. returns can be made.
Yes, Collector will send an updated invoice automatically for Direct invoices, when a part return is done. For other types of invoices (annuity, account), the updated amount will be sent to the customer on the next monthly invoice.
The merchant can extend the invoice due date either in the Partner Portal or via Payments API. Using the API requires that it is agreed with Collector. The extending of due date is only available for Direct invoices and it can only be done once per invoice.
Collector will automatically contact the end customer by email, if the end customer has any overpayments after a return was registered for an invoice. After Collector has received bank account information on My Pages from the end customer, we will pay the money back within one to three business days. For returns on credit accounts and part payments, the return on the order will just reduce the customer’s open balance, and no automatic refund to the end customer is made.