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I am afraid that in addition to a memo on a check, you would need to have a written notarized contractual agreement with the mother which would then have to be filed in the court where the child support award was ordered. The courts deal with child support differently than anything else and they would have to review the agreement to make sure it is conscionable for the best interests of the child before they would approve waiving the support for the future.
Also, if she is receiving government benefit, the court/government could continue to pursue you for payment of your share of those benefits, regardless of the memo on the check. However, if the agreement is accepted by the court, then you would be free and clear.
Thus, you can agree in AZ to pay lump sum for all support, but it has to be approved by the court before it would be valid and binding.
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