As discussed in the Retail Interest Yammer Group, I will post the solution here for the wider audience:
I have experienced a similar scenario where we had a Call Centre that was taking credit card payments offline (essentially like cash.) We configured a method of payment to be specifically used to refund these payments when processing a Return order.
You need to ensure that this Payment method is configured with the 'Default function' set to 'cheque', because Call centre orders can only refund to credit card or cheque Payment methods.
Then configure this Return payment method in the 'Call centre refund methods' form.
Also, in the Call centre parameters -> RMA/Return section, we have the Return payment method set to the same.
I came to this workaround after discussions with Microsoft to understanding how this all works.
Incidentally we do not use a Channel return policy.
This page is useful/important, if you haven't already seen it:
[View:https://docs.microsoft.com/en-us/dynamics365/commerce/call-center-refund-payments:320:237]