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Yes, this is legal. Or rather, what he did to the check is legal. You can still argue what is due and what is not due in Court, but the check negotiation between you and him was lawful.
When you sent the check to him, and wrote RENT on the check, the receiver of the check has a right to place a "reservation of rights" on the check if there is a dispute about it. See AFC Interiors v. DiCello, Ohio 1989, 544 N.E.2d 869,*****3d 1.
Where debtor tenders check to creditor as payment in full for less than amount alleged to be owed on debt, creditor may accept check as partial payment on debts so long as creditor explicitly reserves all rights by endorsing check "under protest" or any legend sufficient to apprise debtor the check is not accepted as full payment on debt, and in so doing, creditor does not thereby prejudice any rights reserved on balance alleged to be due; overruling Seeds, Grain & Hay Co. v. Conger,***** 169, 93 N.E. 892. AFC Interiors v. DiCello, Ohio 1989, 544 N.E.2d 869,*****3d 1. Accord And Satisfaction 11(2). Source HERE.
While this is not quite 100% percent about "accord and satisfaction," he has a right stemming from the above to mark a reservation on the check. That is true. However where he should have applied the check and whether or not it counts as full payment is up to the Judge to decide.
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