I am in a problem

You know the answer to your question. You are also part of the issue and you also should take responsibility for your action. Everyone knows this visa trade is illegal. I am sure even you knew it.

Well said Farhaz 

If you fail to pay the outstanding amount then your "sponsor" can report you to immigration and you will be thrown out of Bahrain quicker than you can ever imagine. If you wish to live here illegally then that is your choice!!

"i buy bahrain visa in 850 bdh from an agent.

It's very obvious by the way you wrote the line above that you clearly knew what you were doing was wrong. Now that it's time to pay up it's a problem? Really now!

What you did is definitely illegal. If you report him, you will be departed for illegal visa trade. If you don't pay for your visa, he can also kick you out. I don't know what to recommend in this case to be honest, but to all readers of this forum; visa trade is illegal and you should NOT practice this at all. You are likely to get into trouble in the long run.