Collections, Business Law question

We are a school who accepts tuition reimbursement from students, recently we ran into where the employer was no longer approving our school. We started talking to the employer and they asked us to send them information on every student and over the course of a year they kept asking for informationand that they would work on it. As a result we stopped the collections process against the students for tuition. After 18 months they sent us a memo that they weren’t going to do anything. Isn’t there a legal principle where if someone takes responsibility for collections they cant just give up and give it back to us? At this point collecting from these students after 18 months would be impossible. Thoughts?