Personally, I don't know. But whoever put the money in is certainly a kind fella. :)

Lucky bastard...

I put it there by mistake. Can I have it back?

MS does make mistakes though. I think 2 years ago the MS store was selling $60 games for $1-$2, and loads of people took advantage of that glitch. 

"We're sorry, there was an error in payment processing and money was added to your account by mistake. 

We will deduct the money from your account and return it to normal.

Account balance -$100.00"

Microsoft Griffin:

Uhhh.... I may have held off on using that. If it was a mistake and they reverse it you've got a bit of a headache on your hands.

I don't think its out of the realm of possibility.  If a bank credits your account some money by mistake its not free money for you.  They can reverse it even if you've spent some of it, possibly giving you a negative balance.  Fake check scams kind of work off of this.

There are stories of PS accounts being hacked, and then the hacker buys a bunch of stuff.  Instead of just revoking the content Sony said 'yeah you actually have to pay for these'.

Obviously these examples aren't exactly whats going on here, but its close.  Clearly there is someone out there in the world searching for a missing $100, probably on the line with MS trying to figure this out.

Guys, I don't have a credit card on my account, so there is no way they can charge me without my consent. Also its been 6 hrs since I made these purchases, and so far I haven't gotten anything revoked. So crossing my fingers.

do the right thing? he's going to get some DLC that at this point is costing the developer like nothing. screw it, not his fault their system is glitching left and right

In the case of the Sony account they locked out the user until he paid the outstanding balance.  Probably nothing is going to happen, but imo trying to figure out where the hell the money came from was the first step.