Albert Barnes Commentary Job 20:18

"That which he labored for shall he restore, and shall not swallow it down; According to the substance that he hath gotten, he shall not rejoice." — Job 20:18 (ASV)

That which he laboured for shall he restore - This means that he will give back the profit of his labor. He will not be permitted to enjoy it or to consume it.

And shall not swallow it down - He will not enjoy it; he will not eat it. He will be obliged to give it to others.

According to his substance shall the restitution be - literally, according to Gesenius, “As a possession to be restored in which one rejoices not.”

The sense is that all he has is like property that a man has, which he feels is not his own, but which belongs to another and is soon to be given up.

In such property, a man does not find the pleasure that he does in what he feels to be his own. He cannot dispose of it, and he cannot look on it and feel that it is his.

So Zophar says it is with the wicked man. He can look on his property only as that which he will soon be compelled to part with. Not having any security for retaining it, he cannot rejoice in it as if it were his own.

Dr. Lee, however, renders this, “As his wealth is, so shall his restitution be; and he shall not rejoice.” But the interpretation proposed above seems to me to accord best with the sense of the Hebrew.