Your questions fall outside the realm and scope of Immigration law and are all labor or employment law related. You should repost at the labor or employment law section, but I suspect they wouldn't know how to answer it due to the immigration law wrinkle (MTR; "illegal").
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Your question is complex and I don't see all the facts here. You need to speak to legal counsel immediately. It seems from what I read that the bonus check is due from the prior employer? If this is correct then they would be able to issue the check provided it was earned by you while you were under valid H1B status with that employer. However, I am not clear about that from your statements.
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In most likelihood the employer is obliged to pay the bonus if it was promised to you and you earned it. However, DOL Wage and Hour Division complaint may not work as the FLSA, with some exceptions, requires bonus payments to be included as part of an employee's regular rate of pay in computing overtime. Professionals usually cannot claim overtime. I agree with Attorney Behar. Your best approach is to contract a Employment/ Labor attorney. BTW DOL will enforce the prevailing wage if you were not paid your full salary.
This response is general in nature and cannot be construed as legal advice, given that not enough facts are known. Any comments offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship. You are encouraged to seek independent and private counseling for a complete review of your case.