Walden University Online degree program sent to collections for unpaid tuition, but I withdrew before charged Online Internet

My husband took the intro. class in 2012, but withdrew before he was charged tuition. I personally worked with the student counselor to verify his withdrawal as I was attending St. Leo University online and had clear understanding of tuition charged deadlines. In Jan. 2017 he started receiving collection notices from a collection agency, but we have no prior notice from the school of any owed amount. They also did not start reporting to the credit bureaus any delinquency until 1 year after he initially enrolled. He was setup through FAFSA and had been awarded financial aid, so the school would have needed to charge the tuition to the financial aid and not directly to my husband. The collection notice is currently for over $2700. There is no explanation for why the school would wait over 1 year to collect tuition, and on top of that for a class that he was withdrawn from. And why is this just now showing up on his credit reports? If he was truly delinquent they would have reported several years ago. Due to this unjustified collection we are now being denied on credit applications. If it weren't for this issue I would have just let the delinquency expire from his credit report, but we can't wait that long.