This occurs because the State automatically assumes that the reason your ex is on Welfare is because you aren't paying your child support. Even providing proof that you have indeed made your child support payments may not stop the State from trying to collect the money from you. (The State figures that if your ex is on Welfare she "obviously" has no money so they won't be able to collect anything from her, but you, on the other hand, have been making child support payments- therefore you must have money, and they're going to try and get it from you.)
Normally when this situation occurs, you have to provide an enormous amount of proof to the State to prove that you were in fact making your child support payments. It is usually an arduous process that is both time-consuming and expensive. As stated above, even after you have provided proof to them they may still insist that you pay for your ex's Welfare costs.
One possible avenue for you is through the State's own rules and regulations. Some States have a provision on the books that when a custodial parent applies for or receives Welfare, the non-custodial parent of the children involved must be notified. Theoretically this provision exists so that the person paying support (read: father) has the opportunity to pay any arrears there may be, thereby eliminating the need for the custodial parent (read mother) from having to go on Welfare.
The State must not only send you notification that your ex has applied for or is receiving Welfare, they must also be able to PROVE that they have attempted to notify you (with a receipt for Certified Mail delivery, for example). Many fathers are duped out of money they don't owe simply because they aren't aware of their rights as fathers, and because they weren't aware of the very well hidden clause stating the requirement that the non-custodial parent must be notified (usually by Certified mail) to appear in order to establish support.
The steps you must take to try and prevent an unjust Welfare recovery action are as follows:
Locate the law that details the responsibilities of the Welfare or Support Enforcement office that is attempting to recover the money from you. Check the Handbook On Support Enforcement or the Federal CSE Home Page to look this up. Another great way to get this information directly is to call the Support Enforcement office in your State and ask anonymously as if you are the custodial parent. In most cases you'll get all the information you need to protect yourself from unfair judgments.
Assemble all the proof you can that you have been paying support, either in part or in full. Get bank statements, canceled checks, money-order receipts- anything that shows that you have been making child support payments. Be aware that most States require that child support payments be made through a Support Enforcement office- any money given, sent, or transferred to your ex that does not go through the Support Enforcement office will be deemed a "gift" and will NOT count towards the support obligation.
As soon as you find out that a collection order is being carried out against you, notify the Support Enforcement office in by Certified Mail that they are in error, and request a hearing (sometimes referred to as an 'Administrative Hearing'). If they do not respond, notify them again by Certified Mail.
At the hearing, present your documentation showing proof of payment towards the support obligation. If you can get an admission from your ex that she was receiving support from you, so much the better. If, however, she denies receiving support and you have proof that you were paying, point this out to the Support Enforcement personnel and show them your proof of payment.
Next, ask for proof that the Support Enforcement office contacted you or attempted to contact you to inform you of your ex's request for Welfare. Undoubtedly they will have a record of when she applied for Welfare (they will ALWAYS have this) and so they should have a corresponding record indicating that they then notified you (or attempted to notify you) as required by law. If they do not have any record of notifying you, produce your copy of the law that requires them to do so, and demand that since they acted illegally, they must stop their unlawful recovery action immediately.
Reproduced below is an example statute detailing the requirement for the non-custodial parent to be notified. (This example is from New Hampshire, be sure to locate the correct one for your State.) Notice that the actual mention of the requirement for notification (Sec. 2.II) is virtually hidden in the statute. This brief, obscure mention is the only section of the statute that even implies that there must be notice served upon the person paying support (the obligor).
§ 161-B:4 Payment of Child Support to the Department.
I. Child support payments shall be made payable through the office of child support enforcement services of the department upon written notice by the commissioner to a person responsible for the support of a dependent child or to a court or administrative agency having jurisdiction over such person that:
The children for whom a support obligation exists are receiving or have received assistance pursuant to RSA 161 or RSA 167; or
A person has applied for or is receiving support enforcement services under RSA 161-B:3, II, and such person has certified to the department that:
The arrearage on the support obligation has accumulated to an amount equal to the support obligation for one month; or
A court, pursuant to RSA 173-B, has issued a protective order enjoining a parent of a child from abusing or interfering with the liberty of the child's other parent, guardian, or legal representative, and that such order remains in full force and effect at the time the person applies for child support services.
II. Notice pursuant to paragraph I shall supersede any court or administrative order to the contrary. Any person who knowingly fails to comply with the requirement of such notice shall be guilty of a misdemeanor. (Emphasis added)
III. Nothing in paragraph I precludes the court, in its discretion, from ordering the payment of child support through the office of child support enforcement services of the department if the court finds upon the motion of any party that it is in the best interest of the child, obligee, or obligor to do so.
Examine your state statute carefully; it may take a careful reading to find the provision covering the requirement to notify the paying parent.