Jamieson, Fausset and Brown on 2 Corinthians 8:12

12. For--Following up the rule "out of that which ye have" (2Co 8:11), and no more.

a willing mind--rather, as Greek, "the readiness," namely, to will, referring to 2Co 8:11.

accepted--Greek "favorably accepted."

according to that a man hath--The oldest manuscripts omit "a man." Translate, "According to whatsoever it have"; the willing mind, or "readiness" to will, is personified [Alford]. Or better, as Bengel, "He is accepted according to whatsoever he have"; so 2Co 9:7, The Lord loveth a cheerful giver." Compare as to David, 1Ki 8:18. God accepts the will for the deed. He judges not according to what a man has the opportunity to do, but according to what he would do if he had the opportunity (compare Mr 14:8; and the widow's mite, Lu 21:3, 4).