Nay, but has not done what He commanded him.
Ibn Jarir said,
"Allah is saying, `Nay, the matter is not as this disbelieving man says. He claims that he has fulfilled Allah's right upon him regarding himself and his wealth.
لَمَّا يَقْضِ مَأ أَمَرَهُ
But he has not done what He commanded him.
Allah is saying that man has not fulfilled for his Lord the obligations that were imposed upon him."
What seems apparent to me of its actual meaning -- and Allah knows best -- is that the Ayah
ثُمَّ إِذَا شَأءَ أَنشَرَهُ
Then when it is His will, He will resurrect him.
means, He will resurrect him.
كَلَّ لَمَّا يَقْضِ مَأ أَمَرَهُ
Nay! But he has not done what He commanded him.
means, He has not done it (resurrected them) as of yet, until the time period has expired and the extent of the earthly life of humanity is complete, according to the lives of all whom Allah has written it to exist from the time they are brought into existence into the world. Verily, Allah has decreed the existence of mankind, and its duration, therefore, when that is finished with Allah, He resurrects the creatures and repeats their creation just as He initially created them.
The Growth of the Seed and Other Things is a Proof of Life after Death
Then Allah says;
فَلْيَنظُرِ الاِْنسَانُ إِلَى طَعَامِهِ