Have any veterans experienced this and were you successful at disputing the claim

My father spent 24 years in the military and they cleared him in the end saying he was good to retire and he served his time. Then later down the road claimed he actually wasnt supposed to retire, that he needed to serve 2 more years and since he used the GI bill for my his children college he has to pay the money back. He has been working another job since to clear the debt. Is there anyway out of this?