Bitcoin is a protocol... just like SMTP.
A bitcoin is a medium of exchange... just like USD.
Bitcoin is NOT a currency!
In it's simplest form it is just payment rails. Stop thinking about is as currency and what is left to regulate?
If a merchant selling a pair of jeans for $250 accepts a 1 bitcoin and customer 2 weeks later asks for a refund with a value of a bitcoin going up to $300. Is he going to get 1 bitcoin back or a value of the jeans he bought? Naturally, the value, which means 3/4 of a bitcoin at the time of the refund. So, how does it make bitcoin a currency. ITS NOT!
What do you think?
No. It does not come naturally. It should depend on the contract between the buyer and seller. Just think about your example in terms of Zimbabwean dollar instead of USD and imagine the same incident happening at the time of Zimbabwean dollar crash.