Jamieson, Fausset and Brown on 1 Corinthians 9:17

17. Translate, "If I be doing this (that is, preaching) of my own accord (which I am not, for the 'necessity' is laid on me which binds a servant to obey his master), I have a reward; but if (as is the case) involuntarily (Ac 9:15; 22:15; 26:16); not of my own natural will, but by the constraining grace of God; (Ro 9:16; 1Ti 1:13-16), I have had a dispensation (of the Gospel) entrusted to me" (and so can claim no "reward," seeing that I only "have done that which was my duty to do," Lu 17:10, but incur the "woe," 1Co 9:16, if I fail in it).