Your case is an example of how scammers can easily scam crypto traders with tempting offers. Perhaps you should be asking yourself why an exchange would let you sell bitcoins at a higher price than the market price. They let you withdraw a few times to build trust in them, and to make you willing to deposit large amounts of funds. Sorry for your loss. But thank you for making this Scam accusation about this matter. This thread will alert many crypto traders who run after such tempting offers.