Uber driver holds wallet hostage for money

We requested a ride on sunday. The driver complained about our short trip. My husband took out his wallet to tip the driver and then the wallet fell out in his car. We called him 5 minutes later and he confirmed that he was in possession of my husband's wallet. He promised to bring it as soon as he finished his shift. Since then it is a struggle. He is now demanding R350 for the wallet. We tried to contact uber but to no avail.