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.
sorry but is not correct, one seller accept for example 250 usd or the EQUIVALENT in btc at that moment. In the other hand following the legal argument of the "refunds only in USD or is fraud" if you pay in btc legally is the same you pay in fresh air, no legal tender, etc... only the goodwill between the parts.
Blackarrow
at last some good news.