Bitcoin was the payment method/medium, not the payment "amount." To put it another way, treat BTC as check/money order/Credit Card... [...]
Bitcoin
is the payment amount and the payment method. Treating BTC as a check/money order/Credit Card is absolutely insignificant to the pertinence of legitimate, fair refunding; when you pay someone for a 100euro piano keyboard (thru paypal for instance) with usd totaling $135, and you decide a couple of hours, or days, or weeks later (it doesn't matter when) that you would like to cancel your order for the piano, they
will return EXACTLY $135 thru proof of your paypal invoice. They
will not return
only $37.20 in that time frame of you cancelling your electric-piano order because the price of 100euro skyrocketed %350.
Actually, that is exactly what they will do. If you purchase that 100Eur piano on your credit card and get charged $135, if in a week when the exchange rate is different you will get 100Eur back at whatever the rate is at the time of your return. In fact, even if the rate is exactly the same you'll have the joy of paying the ForEx fee on both the payment and the refund.