CA saying I must pay back GI Bill

About 4-5 months ago, I started receiving phone calls on my cell phone from a number I didn't recognize. No voice mail was ever left but every day, they would call about the same time...sometimes twice a day. Finally, I answered the phone to a guy that sounded half stoned on weed that was basically telling me I owed $531 to the US Dept of Treasury for my GI Bill. He said I was overpaid but as he went into more detail, it says because I didn't complete classes I signed up for during a time period I wasn't in school.

Now, I've been overpaid by the gov't before and they just took it back, without mercy. I feel this is a scam as I can't imagine the gov't outsourcing their debt to a CA. If they want it, they'll just take it...right?

Anyways, I ended up telling that guy to stop calling me but clearly it didn't work as the calls kept coming. Granted, they were always at the same time everyday (mon-fri) but never any voice mail.

2-3 weeks ago, I answered the phone call from them after I spent all that morning reading over this website. I was ready for them and I informed the lady that I was who she was looking for and I require a verification of debt. I also told her I expect the phone calls to stop till no sooner than 30 days after I receive their mail on this matter.
Sure enough, no phone calls since then and today, I received a letter from them. It just states the amount I owe, their company and who they are collecting for. That's it.

So my question is that since this isn't a credit card or loan account, when I send a followup Debt Validation letter, how will this play in regards to the account history? Is this alleged debt treated differently than say a Home Depot credit card?

Thanks in advance for any info/assistance. This site is fantastic!