in job god told Satan you can only hurt him physically, but not kill him. Satan obeyed even through he was thrown out of heaven, he is still under his power and if it was up to him he would have killed job, because he was jealous of him. so he asked god if he would strike your hand against he would guarantee he would curse him in his face. If satan killed him since he was his servant, god would severely punish him and satan was afraid to say anything because of that.