If me and my ex agree to sign a paper stating she will never raise my child support over $400 no matter the circumstance. Will this work? I had a child support review and they told me to ask a lawyer or attorney.
You can't make such an agreement; it wouldn't be binding. It isn't fair to the child.
However, your solution is very simple: If Mom doesn't want more than $400 per month, then she doesn't have to TAKE it. If ever in the future your child support is increased, Mom can simply refund the difference (to you) each month.