No, it is not legal: a signed written agreement is a contract, and it binds *both* sides. As long as you honor your obligations, they must honor theirs--which in this case, means accepting the payment plan and not trying to collect other ways. This may have been an error on their part (someone didn't "get the memo," so to speak), so the first step is to contact them and ask them to return the money because you had an agreement about payment. If they won't fix the situation, then you could sue them for breach of contract to recover everything over and above whatever amounts you would have paid to date under the agreement.