Exhusband wants to relinquish rights

My ex husband wants to relinquish rights. My husband wants to adopt my daughter. My ex wants me to send in documents to the court that all past due child support and all future child support be waived. I don't have a problem with this but I just wanted to make sure that it was legal. I don't want it to seem like I am selling my daughter. My ex hasn't seen my daughter in several years and has not paid any support in over 3 years. Any ideas?:confused: