Parents cannot abrogate their child's "right" to support. Therefore, a court can order support at any time, upon the request of a parent, regardless of any prior agreement of the parents.
However, the parent receiving support cannot simply avoid employment in order to force the other parent to pay more support, so if your husband can show that the recipient parent has the talent and ability to earn $X, and is unwilling to work, then the court can impute income to the recipient parent which will reduce the paying parent's support obligation and simultaneously "encourage" the recipient parent to seek work.
Note: I don't minimize the difficulty of proving elements necessary to impute support. However, if the recipient parent was working and has suddenl quit, then that parent's prior employment wages and benefits would suffice as proof to impute income, absent proof from the recipient parent that she was involuntarily terminated and no other work is available in the job market in her locale.
Yes, the ex can take him back to court. And, unfortunately, "appropriate" child support, per federal law, must be based on the parents' income and not the "needs" of the child. A receipt for daycare can be obtained only if the other parent is receiving add-on daycare costs and not obtaining any daycare services. But, you can't get the receipt in advance -- you have to observe the other parent cheating and then initiate a support modification proceeding.
As for the 50/50 custody issue, if you have that in the orders, then you should be exercising those orders scrupulously, and if you're not, then you are settingyourself up to losing that arrangement. And, if your orders don't actually provide equal parenting time, then you don't really have 50/50 custody.
Note: I'm actually "on your side." I believe that the present child support laws border on the insane. But, the law is what it is, and it's on no one's political agenda at the moment to change these laws -- so, if you don't learn to play (and pay) by the rules, then the child support train will run right over you like you weren't there at all.
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