Go back and re-read the whole chapter. In verse 6, you'll see that God has the power to keep a person from sinning against him. Also, in reading that chapter and the following chapter, you'll see that God had a purpose in the entire incident, start to finish. Abraham was one of God's prophets, one of those he speaks with. If God had a problem with Abraham's conspiracy with Sarah, he could have spoken with him about it, and he could've stopped him from sinning. He chose not to, for his own purpose.
BTW, when you read the whole chapter, note that Abraham and Sarah had their conspiracy going on for some time, and apparently used the same scheme more than once. God's ways are above our ways.