H1b reopened and approved-no longer with the employer

I was with a consulting firm in 2018 and my H-1b was picked and got denied that year with specialty occupation reason. Yesterday, I was notified by USCIS email that my denied H-1b was reopened and got approved after my ex-employer won a case against USCIS and got denied cases overturned and mine was one among them. I am no longer with that employer. I am currently on F1. My current employer is asking for the receipt notice before making any decision. I am almost certain that it is a COS H1B application. Can I legally work with my current employer till I get my receipt and initiate a transfer?