An employer holds the passport of an employee after accepting his resignation

Can someone here help me, please!

My brother is in Dubai and finished his 2yr contract. He doesn't want to renew anymore so he decided to submit his resignation and stop working.

His employer agreed, however, they refused to give him his passport. They also told him that once he gets his passport, he will be arrested as they filed a case against him.

In line with this, may I ask anyone's kind assistance in this matter? My brother needs to work as he doesn't have a visa and he doesn't want to return to his old company.

Also, I would like to ask if he is entitled to gratuity even if he finished his 2yr contract only?

Many thanks in advance!