Originally Posted by greenbaggins

Another problem here is the definition of merit. There are three kinds of merit: snip...

The third kind of merit is usually described as "improper," because it doesn't work like the first two kinds of merit. This is "pactum merit," or "covenantal merit." This refers to an action that does not, in and of itself, deserve the reward, but will obtain the reward because the people involved have agreed that it will. ....
Reformed theologians also refer to Christ's work as condign and pactum merit. It is condign because Christ's obedience was perfect, and the law says, "Do this, and you shall live." Christ did it, and all who are in Him live. His condign merit is imputed to us by the instrumentality of faith. But the Father and the Son also agreed in eternity that the Son would do this, so it has that aspect of pactum merit, though not the improper aspect (since the action measures up precisely to the consequence).

Now, in Piper's formulation, he wants to make sure that Adam would not have earned or merited God's favor. If he is rejecting condign merit, fine.
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However, Piper's formulation seems to reject any kind of merit, proper or improper. I would therefore disagree with Piper's formulation.