Ah, but the agreement was apparently made before the child was emancipated, and thus there was likely the option to pursue the court ordered child support at the time the agreement was made. Giving up that option in exchange for the agreement would constitute valid consideration.
The "agreement" might NOT be worth the paper on which it is written.
Many times these "agreements" are adjudicated to be nothing more than "gifts", despite the fact they were reduced to writing.
If that occurs, the person thinking he/she was paying child support, is found to be merely a generous donor of cash gifts.
It never ceases to amaze me why people persist in agreeing to things outside of the court, only to discover the only one that got cheated was the cheerful "giver".
These agreements sometimes attract the attention of the various tax authorities, as they violate the statutory limit on giving.
I believe the IRS gift limit for 2018 was $15,000.
The little schemes and scams we employ THINKING we are avoiding the gubmint bureaucracy usually don't end well for us.
If you give any one person gifts valued at more than $10,000 in a year, it is necessary to report the total gift to the Internal Revenue Service.
The person who receives your gift does not have to report the gift to the IRS or pay gift or income tax on its value.