When your buyer requested the return, there should have been a link there for the buyer to print out the unpaid label with your return address on it. Amazon also sends buyers an email with that link too titled "Your return of your Amazon.com order number....".
Ask your buyer to print out the label with your address on it and purchase the postage at the carrier and you will reimburse the buyer when you receive it back.
I understand that you feel under a lot of pressure but the buyer has to take some responsibility to return the item before they get their refund.
If worse to worse and the buyer files a claim, Amazon should at least have your back for doing what was necessary to help your buyer with the return. You can lead a buyer to a return but you can't force them to go through with it.
If your buyer requested the return which you approved, they should have received an email from Amazon like below. The email has a link to the buyer's account where they can print out the shipping label and pay the postage to be reimbursed later.
Thanks for using Amazon's Online Returns Center!
For instructions on how to print your return authorization and mailing label and prepare your package for return shipment, please visit the following web page: