Am I legally abide to be going back to the original rent or should something else be applied to this matter if nothing was signed?

I have a couple of questions of concern. I am a renter in a condo that is privately owned. When I first moved in, I signed a month to month agreement for $400 which included utilities. Then after a couple of months, he brought my rent down of $100, but he and I never signed an agreement of my new rent price. It has been now 6 months and now he is trying to go back to the original rental agreement of my rent of $400, even though I have been giving him only $300. As I mentioned before, since the rental price changed, and no agreement contract was signed. Any information would be greatly appreciated.