How is the refund amount calculated?

Given the purchase will be splitted into 4 payments, payable every two weeks, when you initiated the refund, Afterpay/Clearpay will net off the refunds from the future payments first.

For example, if the total amount to be paid is 100 USD, which is splitted into 4 payments, 25 USD for each, and your account is only debited with 25 USD for the 1st payment, then you initiated the refund of 80 USD, what happens will be, the last 3 outstanding payments will be cancelled, and you will get 5 USD paid back to your linked bank account.