Partial.ly Payment Plans can only be canceled and refunded through your Partial.ly Merchant Account.
Login to your Partial.ly merchant portal and select the payment plans section. Select the payment plan from the list containing the payment you want to refund. On the payment plan details page, click on the Payments tab to find the Payments section. Here, you'll see a list of all the payments for the payment plan. Simply select the refund button next to the payment you want to refund.
You can refund the full payment, or any amount less than the full payment by typing the value in the text box.