New Zealand Law
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If you are said to have lent the father in law this money, then you can sue for this in the Disputes Tribunal. He may find it difficult to defend this if it was an essential element of the settlement. However you would need to consider the terms of the loan, and make formal demand first.
You cannot sue him then, and you cannot sue your ex wife because the agreement settled all issues. You would not be able to recover any money from him, because he supposedly lent you the money, and on repayment this would discharge all liabilities
Sorry I cannot suggest any remedy