If they clearly stated below facts in agreement,

1. Main currency is USD or other real currencies.
2. They offer BTC acceptance as a side option. In other words, It's just an exchange service between BTC and USD.
3. Therefore, BTC payment do not construct a direct payment.
4. If you paid in BTC, amount of BTC you need to pay is determined by the exchange rate of the time of payment
5. Likewise, If you ask a refund in BTC, amount of refund in BTC will be determined by the exchange rate of the time of refund.

I think it's done deal unless they use unrealistic exchange rate.

Let's say you bought a very fast computer with your one kilograms gold bar.

CASE1. Merchant originally accept 1kg gold for 1 computer deal.
CASE2. Merchant originally accept USD for 1 computer and they have an additional service which is "Take some gold and exchange it to USD"

In Case 1, If you give them 1kg gold for a computer, you will get 1kg gold refund.
In Case 2, If you give them 1kg gold for a computer, you will get USD refund.
And if you wish to receive your refund as gold, exchange rate at the time of refund will be applied to determine how much gold you will get.