well I don't think they should give him the money right now with a new contract, but they should throw in some incentives to let him have the opportunity to make some more money this season.
ie...if you go out there, and play hard, put up good numbers, and stay out of trouble off the field, we'll give you bonuses, under categories x, y, z on top of your base salary this season...and if at season's end you've done all that, we'll sit down a negotiate a long term deal for you.
I think that would be fair, and it would motivate Brandon to go out and play hard, and it would allow the Broncos to retain Marshall.