You have the right to ask to sign a lease or other agreement. You have the right to ask for a receipt when paying cash. You can most certianly sue him - and could even win if you can show damages - such as not being able to stay in a place as agreed (present lease to the judge) or that you were evicted after you paid and didn't get to enjoy the lease period you paid for (present receipt for cash to judge).
Anyone can sue for anything. The real question is: did you get to stay in the place for the period that you paid for? If the answer is yes, you don't have damages. If you paid for a month and were thrown out against your will by either him or his landlord (since legally you are HIS tenant, not his landlord's tenant) before the month was up, you'd be able to sue for the amount of the rent commiserate with the time you didn't get.Can I sue him for the rent back?
In matters of court, verbal contracts and cash payments are not the way to a successful suit in most cases. You'll say you paid amount A - he'll say you paid amount B. The judge will ask for your proof of amount paid. What will you hand to the bailiff? If you've got a receipt for $100, he'll say the rent was $200 and you only paid half. Got a lease agreement that says otherwise? Got a history of documentation that you only paid $100 in every prior month? See where this is headed?

