What should I do now after taking money that fell from its owner without his knowledge on a farm where I was working, and I cannot find the owner of the money?

First, you must repent to Allah Almighty for the theft, by feeling remorse and resolving not to return to it in the future. You should have been trustworthy and alerted the man to the money that fell from him, rather than stealing it. If you find no way to return the money to the man, then you must give it as charity on his behalf.