Kerr was probably wanting to pay him max.
Kerr was the one leading the charge to trade Amare to Cleveland. Just because Kerr was friends with Happy doesn't mean Kerr was going to open up the bank.
I've heard nothing to suggest Kerr had any qualms with how the Suns planned to negotiate with Stoudemire. That was the one thing Sarver/Kerr agreed on. Maybe the only thing.