If I want to get a refund from a seller I should receive the full amount of Bitcoins I paid to the seller correct?
I'm reading your question to be a refund in btc versus a refund in fiat equivalent.
I agree with NUFCrichard above...given that btc can have spikes and fiat is more stable atm - I think it should be fiat equivalent for time being.
Otherwise if the price of btc has changed a fair bit, there will be an unhappy/unwilling side.
Not too many people would be happy getting a 2 btc refund ($600 fiat equivalent) when they paid 2 btc ($2000 fiat equivalent). But on the hand, they'll be expecting their full 2btc when the price has gone the other way (in their favour).
It's easy to stand your ground and argue the point when the price has gone in your favour - but regardless, I think people should be fair.