Well, that depends on in what currency the merchant denominated that jeans. If that was $250 but as an alternative payment method he/she accepted 1 BTC (at the then current $250 exchange rate) then he should refund $250 worth BTC. If he sold that Jeans for 1 BTC what was worth $250 in that time then he/she should give back 1 BTC even if it went up to $300 in the meantime.