Charles Ellicott Commentary Deuteronomy 15:10

"Thou shalt surely give him, and thy heart shall not be grieved when thou givest unto him; because that for this thing Jehovah thy God will bless thee in all thy work, and in all that thou puttest thy hand unto." — Deuteronomy 15:10 (ASV)

You shall surely give. —“Even a hundred times.”

Him. —“Between you and him alone.” (Compare to Let not thy left hand know what thy right hand doeth in Matthew 6:3). I have thought it worthwhile to borrow the comments of Rashi on these verses (Deuteronomy 15:7–10) almost entirely, to show how well the Jews have understood the true principles of Christian charity from the law of Moses. This people has always been remarkable for kindness to its own poor.

For this thing. —Literally, this word, or this promise. And Rashi observes, “Even when you have promised to give, you will receive the reward of the promise as well as the reward of the deed;” and we may compare St. Paul. If there be first a willing mind, it is accepted according to that a man hath, and not according to that he hath not (2 Corinthians 8:12).