I don't know what it takes to sell a gun wherever you live. Here in Florida, you can sell to individuals without involving the government or someone with an FFL. If that's also the case where you live, you would probably make out better financially to sell the M&P to an individual buyer. You might even find a good trade for/toward the 9mm you want. Look for a local gun forum or buy/sell/trade website and list it there. Or if your favorite range has a newsletter, there might be a B/S/T section there. If you don't mind involving an FFL, there's the B/S/T section here, and there's always gunbroker.
If you can't do that in your state, or if you prefer the quick-and-easy trade to your local gun shop, by all means do the trade with your coworker. You'll be helping a buddy and supporting your LGS.
Just make sure that one day you aren't going to wish you had kept the M&P and saved up for the 9mm to boot.