Seller need to specify what is the base currency and refund should stick by it. tl;dr for previous posts.

For example, if the seller advertises the product as $250usd and accepts $250 in bitcoin @ that moment's exchange rates, when refund, if the buyer paid in btc, the buyer should get back $250usd worth of btc @ the current exchange rates.

Unless you are bitmain who sold S1 only with btc.