The lien does not prevent the house from being sold. She just gets her $3000 paid. Transferring title to your name alone does not impact the lien. It will confirm the remaining equity is yours.
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If the ex-wife is not responding, so that you cannot get the released, you may want to consider calling the attorney who filed the lien (his or her name is hopefully on the lien document as having been prepared by them), and ask that they give you the correct amount to get the lien release, tell them you are selling the property and will provide them with a payoff. Ask them for a payoff good through the date of sale. If you cannot find an attorney who was involved in placing the lien, your boyfriend may need to go back into the Court that authorized the child support lien to help get the lien released. The Court may well be able to Order the release of the lien in exchange for payment.