C. F. Keil and Franz Delitzsch on Ecclesiastes 2:18

Ecc 2:18 “And I hated all my labour with which I laboured under the sun, that I should leave it to the man who shall be after me;” i.e., not: who shall come into existence after me, but: who shall occupy my place after me. The fiction discovers itself here in the expression: “The king,” who would not thus express himself indefinitely and unsympathetically regarding his son and successor on the throne, is stripped of his historical individuality. The first and third שׁ are relat. pron. (quem, after the schema egymologicum עמל עמל, Ecc 2:11, Ecc 9:9, and qui), the second is relat. conj. (eo) quod. The suffix of שׁאן refers to the labour in the sense of that which is obtained by wearisome labour, accomplished or collected with labour; cf. כּח, product, fruit, Gen 4:12; עבודה, effect, Isa 32:17.

How this man will be circumstanced who will have at his disposal that for which he has not laboured, is uncertain.