Did this take place actually, or did it take place in the vision? ‘And he brought him forth abroad, and said, Look now toward heaven.’ Many expositors have felt this means he put him right back into a vision, but it doesn't actually say that.
Sometimes people say, when you read Genesis 15 and then you read Galatians 3:16, you wonder if Paul is playing a trick, and treating what is plainly a collective seed, referring to all the seed of Abram in Genesis 15 and making it a singular noun, speaking of Christ. Is he twisting the passage? Of course not. Both are true. When you see the seed of God, of Abram, the true people of God, you see Christ. It is both collective and singular. It is Christ and it is all saved people who are in him through the covenant of grace. Abram knew Genesis 3:15 and he knew Genesis 12:1-3, and he knew that that numerous seed was for Christ, and through Christ, so he was thinking of the one supreme seed as well as all the little stars. You really cannot understand the passage without understanding that: that Abram sees both.