I got a job offer but was asked to pay visa costs. Is this legal?

I got a job offer from one UAE company but I have to pay visa fees as a security which they will return after six months. They will provide me employment offer letter and at the same time I need to pay them visa fees.

I am a bit confused that the offer letter is a legally binding document to claim in future if they will not provide me the job.

Kindly help me to solve my problem. I am on a visit visa in Dubai.