Returning prepaid rent question

I manage a manufactured home park. The tenants own their own homes, but rent the space from us. One of our tenants sold their home, mid month, and the buyers signed the lease assumption for the space mid month. I returned the space rent to the seller for the remainder of the month, and billed the buyer for that amount. Now the buyer is furious, as he says that this wasn’t mentioned in the sale or in escrow, and doesn’t want to pay. I have explained that we are not involved with the sale of the unit, nor escrow. If there was a problem there, perhaps he should contact the real estate agent or the seller.  However, I have a signed lease from him beginning mid month, and the seller formally requested this prorata prepaid rent be returned to him. Is there any statute relating to this that I can share with the new buyer?