Whether Microsoft has a right to get their money back is another subject - the point here is that this letter points to a corporate culture that has low regard for how people relate to brands. Or, in the words of Dale Carnegie, "How to Win Friends and Influence People."
They had the time to create, formalize and establish bureaucratic processes, but managed to snatch defeat from the jaws of success by not sending this letter:
We screwed up and overpaid your severance. It gets worse - you'll have to pay taxes on it, too.
So, you can either keep the money and do with it what you think is best or you can send $xxx.xx back and we'll get a new statement out to you.
Personally? I'd keep the cash. Spend it on something cool (not iTunes) and get this economy rolling. Ok?
Sweet jiminy - they could have had a PR bonanza.