The gain is the difference between the value when they got it and the value when they sold it, so probably nothing if they do both on the same day. I guess you

The gain is the difference between the value when they got it and the value when they sold it, so probably nothing if they do both on the same day. I guess you may have made a gain when you gave the Bitcoin away, since it's treated as a disposal at the market rate... maybe? I don't know the rules for this. (Regarding speed of selling - there's no short-term CGT rate in the UK, but there are rules about quick turnaround that may apply. Consult an accountant.)