After he was traded at the deadline from Buffalo to Minnesota, he elected to head back to Buffalo with a five-year deal worth $25 million per Elliotte Friedman (and many more).
This is a great deal for the Sabres on a couple levels. First off they get a consistent winger in Moulson whom they know what he's capable of doing, having seen him for a few months during last season. Secondly, it helps them get closer to the salary cap floor while adding a pretty good player in the process.
Then don't forget that this is clearly a sign that Moulson wants to be in Buffalo, which is pretty important given where they are at in terms of the franchise's development, as in rebuilding.
Moulson had 11 goals and 18 assists in 44 games for the Sabres last season after being traded in the Thomas Vanek deal. He had 23 goals and 28 assists total over his three stops (Islanders and Wild too). A three-time 30-goal scorer, Moulson should help the Sabres keep some punch while they develop their young players, to which they'll be adding more in the coming years.