John Lightfoot on Luke 19:8

[The half of my goods I give to the poor.] I. A distribution amongst the poor of these goods that had been ill got was necessary. In Sanhedrim there is a discourse of restitution, and distribution of dishonest gains, especially what wealth had been got by merchandise of fruits of the seventh year, which are forbidden. And this is the form of restitution: "I, N., the son of N., scraped up such a sum by the fruits of the seventh year; and behold, I bestow it all upon the poor."

II. Alms were to be given to the poor out of wealth honestly acquired: but according to the rules and precepts of the Rabbins, they were not bound to bestow above one fifth part. "As to what help is to be afforded by mammon, there is a stated measure; a fifth part of his mammon. No one is bound to give more than one fifth." And they say, "That it is decreed in Usha, that a man should set apart the fifth part of his estate according to the command."

The fifth part was so stated and decreed, that, 1., so far they ought to go upon the account of a command. 2. No man is bound by the law to go further. But, 3., he may do more, if he please, on his own accord. Which this Zacchaeus did in a large and generous measure. The restitution of fourfold for his sycophancy agreed with the law about theft.