I ran across this in a case I had and thought it was imporant to get this information out to the public.
In the State of Texas you cannot agree to modify child support outside of court. It doesn't matter if the agreement is notarized, signed in blood or written in stone, you cannot do it.
The only way child support can be modified is through a court order.
What does this mean? It means that if you and your ex agree that you should only pay $100 a month for child support (and put that agreement in a signed, notarized document) because you lost your job, but your order states that you should pay $500 per month, after a year of this you will be in arrears (owe back child support) to the tune of $4,800.
It does not matter that you had a written agreement with the mother. Only the court can modify child support. If you are in this situation then you should spend the money and get an agreed modification order signed by the court.
These can be obtained relatively cheaply and you can do it yourself. I will provide a link below for forms to use in Texas.