I think there is problem for using BTC as a medium of payment in daily life.
If I used 5 BTC to purchase a chair weeks ago,
and BTC rise from 100 USD to 200 USD,
now I want to refund for some reasons,
how many BTC could I get back?
I don't see the problem. Until bitcoin use is widespread enough for BTC prices to be stable, goods and services will continue to be priced in fiat currencies. So you'll pay based on a fiat price, converted to BTC, and when you get a refund you'll receive the fiat value, converted to BTC.
There's no other way it can work right now, but again, I don't really think it presents a real problem. If you don't want to risk losing your bitcoins, then every purchase made with bitcoins needs to be matched by buying more bitcoins to cover it, that's all.