Charles Spurgeon Commentary Job 1:12

"And Jehovah said unto Satan, Behold, all that he hath is in thy power; only upon himself put not forth thy hand. So Satan went forth from the presence of Jehovah." — Job 1:12 (ASV)

And the LORD said unto Satan, Behold, all that he hath is in thy power; only upon himself put not forth thine hand.

Satan could go so far, but no farther; there is an only in the permission granted to him: Only upon himself put not forth thine hand.