For if I do this thing willingly, I have a reward. — The previous words, “Yea, woe is unto me if I preach not the gospel,” are a parenthesis; and now the writer proves the truth of his assertion — that the necessity of preaching the gospel deprives the mere act itself of any grounds of boasting — by showing that if there were no necessity there would be a ground of boasting. The argument is this: — Suppose it to be otherwise, and that there is no such necessity, then, by voluntarily undertaking it, I have a reward. The undertaking it of my own free will would entitle me to a reward. But if (as is the case) not of my free will, but of necessity, then I am merely a steward — a slave doing his duty (1 Corinthians 4:1; Luke 17:7-10).
A dispensation of the gospel is committed unto me. — Better, I am entrusted with a stewardship.