Two questions come to mind:
First, are the payments you send an amount they indicated they would accept?
Second, did they in fact accept the payments, or did they return them to you?
If they accepted them, and they are in an amount they indicated they would agree to, then i think you have an argument that they should not be able to garnish you. Most courts in Ohio will not allow them to garnish under such circumstances.
As always, seek counsel for further analysis.