Since it appears we have verified cases where people received a USD check as a settlement, when they
- Requested a BTC refund
- Didn't request a refund at all (through the form they issued)
For such people, the check is thus just an unsollicited donation from Hashfast.
I was wondering, how long do you have before the check expires? I think in france you have 1 year to cash it in...
So just keep the check, resolve the dispute by getting a full BTC refund as promised in the ToS you agreed to.
And when you have the BTC back, cash the check in to cover for the hassle!

You will likely be required to surrender the uncashed draft or it would be deducted from the settlement.
In the US, in general, a draft (check) is only good for 6 months before it expires and becomes a 'dead draft'; please note that each bank follows different policy however the aforementioned is the usual.