Employment agreement in h1b?

2 months back I got a offer from a company xyz. I accepted and signed the employment contract which says employment agreement starts the day I signed the employment agreement. After 2 weeks before the file the h1b transfer petition, I decided to join a different company abc and informed xyz that I'm not willing to join xyz. They(xyz) didn't respond me for week (looks like they were waiting for something from client) and after a week now they called me and saying I have to join the company otherwise I have to reimburse the cost spent by employer as per employment agreement. Note still haven't filed the h1b transfer petition and I haven't joined the company. For h1b employee before filing h1 transfer, is the employment agreement is valid?