Jamieson, Fausset and Brown on 2 Corinthians 8:9

9. ye know the grace--the act of gratuitous love whereby the Lord emptied Himself of His previous heavenly glory (Php 2:6, 7) for your sakes.

became poor--Yet this is not demanded of you (2Co 8:14); but merely that, without impoverishing yourselves, you should relieve others with your abundance. If the Lord did so much more, and at so much heavier a cost, for your sakes; much more may you do an act of love to your brethren at so little a sacrifice of self.

might be rich--in the heavenly glory which constitutes His riches, and all other things, so far as is really good for us (compare 1Co 3:21, 22).