Approval of previous employer before switching to a new employer

[demiGod15]

I have a valid H1B visa and want to switch my job to a new employer but the problem is in one of the document of my current employer its mentioned that I can only change the job if the company approves it else its not allowed. Offcourse I do understand that once my visa transfer is done I have to officially announce to my current employer about the switch and server notice of 15 days.

But what happens in this condition, where its clearly mentioned that I have to get approval of the company to change my job? the worst part is I have signed this document but would like to know what are impacts if I do? Is it safe or will it I end up in a legal notice from my current employer.

Please suggest.