C. F. Keil and Franz Delitzsch on Ecclesiastes 2:19

Ecc 2:19 “And who knoweth whether he shall be wise or foolish? and he will have power over all my labour with which had wearied myself, and had acted wisely, under the sun: this also is vain.” או...ה, instead of אם...ה, in the double question, as at Job 16:3. What kind of a man he will be no one can previously know, and yet this person will have free control (cf. שׁלט, p.641) over all the labour that the testator has wisely gained by labour - a hendiadys, for חכם with the obj. accus. is only in such a connection possible: “my labour which I, acting wisely, gained by labour.”

In view of this doubtful future of that which was with pains and wisely gained by him, his spirit sank within him.