I had something similar. An item wasn’t as described – the seller was great and agreed an instant refund and return. She also offered to pay my return postage.
I got the item refund straight away, but she contacted me to say that she couldn’t send the postage refund as the dispute had locked her account. I was away at the time, and before I had time to reply or close the dispute I got another email from her saying ‘she had sorted it out’ and the postage refund came through as well.
So – not much help – but yes it does appear that a dispute makes refunding postage difficult, it sounds as though your seller is genuine – but there is obviously a way around it somehow.