This should be governed by the terms of your support agreement. If it isn't, it needs to be. Consult the attorney that helped you get that agreement in the first place.
This answer does not constitute legal advice and does not establish an attorney-client relationship.
What does your divorce decree/order say? This should spell out how this is handles. If it is silent that as long as this is not costing you any additional monies, your “ex” appears to be making things easier on you by cutting down on the back and forth payments by paying you the net amount owed.
DISCLAIMER: David J. McCormick is licensed to practice law in the State of Wisconsin and this answer is being provided for informational purposes only because the laws of your jurisdiction may differ. This answer based on general legal principles and is not intended for the purpose of providing specific legal advice or opinions. Under no circumstances does this answer constitute the establishment of an attorney-client relationship.
I'd have to agree. As long as it comes out the same in the end, what's the beef. He's paying, something a lot of others aren't doing. He's making it easier for you, something a lot of other aren't doing. Take comfort that you are both taking care of the business of taking care of the kids.
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