As I see it, the challenge with offer sheets isn't a future offer sheet in retaliation, it's that you might lose at least one trading partner for a while after you do it. And, who knows, others might not want to trade with you because of their distaste for it. Speculating, of course, but I think the stakes for Murray are high because the dude loves to trade, and risking a trading partner or partners over a player is a maybe a larger gamble than over-paying for a player or two.