It is if you sign it. Every owner has the right to run his/her business according to his/her choice no matter how crazy it looks or sounds to you (as long as it not unlawful of harmful to all parties involved). If it's in your contract then you either have to honor it or leave.
Usually losing 50 lbs should not affect your skin tightness, but it all depends on your genetics. Just wait and see how your body will look after the weight loss and then you can evaluate your situation. Good luck!