Deceitful work - Work which deceives and disappoints the worker; in contrast with the "sure reward" of the second clause.
Omit "shall be" and render, "but he that soweth righteousness worketh a sure reward."

Worketh a deceitful work - An unstable work; nothing is durable that he does, except his crimes.

The wicked worketh a deceitful work,.... Such a wicked man as before described; that neither enjoys the good things of life he has, nor suffers others to enjoy them; and all to accumulate riches, which are deceitful and perishing; and who abstains from meats, which God has created for use, under a pretence of religion, and so deceives his own soul; and indeed every sin which a wicked man commits is a deceitful work; it promises him that pleasure, or profit, or liberty, which it does not give him, and in the issue is the ruin of him; and so all false doctrines, propagated by deceitful workers, are deceitful works, by which they deceive the simple, and at last themselves; they "obtain a deceitful reward of their work", as Gussetius (w) renders it;
but to him that soweth righteousness; does acts of beneficence and liberality; see 2-Corinthians 9:9; and all other good works, or works of righteousness,
shall be a sure reward; according to what a man sows, and the manner in which he sows, so shall he reap, 2-Corinthians 9:6; or, "a reward of truth"; instead of being given up to believe a lie, he shall receive the love of the truth, and abide in it, which will bring him to eternal glory and happiness; he being chosen to it through sanctification of the Spirit, and belief of the truth, 2-Thessalonians 2:10; and, instead of a deceitful reward, shall have a true, real, solid, and substantial one.
(w) Ebr. Comment. p. 692.

He that makes it his business to do good, shall have a reward, as sure to him as eternal truth can make it.

a deceitful work--or, "wages," which fail to satisfy, or flee away (Proverbs 10:2; Proverbs 23:5).
sure reward--or, "gain," as from trading (Hosea 10:12; Galatians 6:8-9).

Man consists of body and soul. In regard to both, benevolence brings its reward, and hatred its punishment.
The godless acquires deceptive gain;
But he that soweth righteousness, a true reward.
Jerome makes 18b an independent clause, for he translates it as if the word were written וּלזרע; the Syr. and Targ. also, as if אמתּו שׂכרו (his fidelity is his reward). But according to the text as it stands, עשׂה extends its regimen to both parts of the verse; to make is here equivalent to, to work out, to acquire, περιποιεῖσθαι, as Genesis 31:1; Jeremiah 17:1, etc. The labour of the godless has selfishness as its motive, and what he acquires by his labour is therefore "delusive gain," - it is no blessing, it profits him not (Proverbs 10:2), and it brings him no advantage (Proverbs 10:16). He, on the contrary, acquires truth, i.e., a truly profitable and enduring reward, who sows right-doing, or better: good-doing, by which we also, as the biblical moral in צדקה, think principally of well-doing, unselfish activity and self-sacrificing love. Hosea 10:12 speaks of sowing which has only צדקה as the norm; and how צדקה is understood is seen from the parallel use of חסד [piety]. The "true reward" is just the harvest by which the sowing of the good seed of noble benevolent actions is rewarded.

Worketh - A work which will deceive his expectation. Soweth - That worketh it with constancy, and diligence.