Typically a bank draft (Check) of which one is attempting to buy another off, in the case of a settlement, will be a 'Conditional Draft' (should have something on it to the effect of 'For payment of refund in full')
Cashing one of these types of checks will be viewed by the courts as an accepted settlement; a draft is a signed contract.

The flip side is if you did not request a refund, specifically have not provided them with any signed documentation to that effect, paid in bitcoins, and the draft has nothing resembling a Conditional then they just sent you a check...

Personally, I would cash it and demand a refund of my BTC as per contract.  If their accountant goofed and sent you a check in error, then they need to get a refund from you.

Does not your contract state to be reimbursed in BTC?

EDIT:  Is their not a cancellation clause in the sales agreement that permits them to cancel a transaction at any time?  A business has the right to refuse service to anyone at any time.