Assuming USA and that you had a standard work for hire agreement, then no, you cannot sell it as your own without getting a release and probably having to pay b

Assuming USA and that you had a standard work for hire agreement, then no, you cannot sell it as your own without getting a release and probably having to pay back whatever salary you received when working on it. The work/code you produced belongs to whoever paid for it. You could rewrite from scratch if you wanted to though.