I'm not an expert on US law, but where I'm from you can cash a cheque & still remain able to claim the remainder of what you claim to be owed. 

You have to reply to them promptly in writing (make sure to date and photocopy the letter before sending it) saying "I have received your cheque as partial payment against the $xxx/BTCxxx owing to me as a refund from non-delivery of my [product name]. Please promptly pay the remainder outstanding, which is $xxx/BTCxxx."  If you want you might also give them a deadline after which you will charge (a reasonable) rate of interest.  Obviously, if you're claiming BTC back you will need to show how you have calculated their USD payment against the BTC balance.

Meanwhile, cash the cheque before they realize what you've done & stop payment.

I'm not a lawyer, but i did study fair trade law to 300 level at uni & there was case law supporting this. I'm sure there will be a similar law/precedent in the US but you should probably check to make sure. Again, not 100% positive about the US, but this worked in a case even where the cheque was sent with a note saying it was "final payment". It could be a way to play Hashfast at their own game.