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The short answer is no. Your ex cannot do this without going back to court and having the marriage settlement agreement changed. However, outside of the legal spectrum, he is probably just trying to be difficult because he does not think that you will spend the time or money to get him back into court over this issue.
Unfortunately, the only way this issue can be addressed is to get back in front of the judge and make your ex accountable for his breach of the marital settlement agreement.