John Lightfoot on Matthew 19:21

[Sell that thou hast, and give to the poor.] When Christ calls it perfection to sell all and give to the poor, he speaks according to the idiom of the nation, which thought so: and he tries this rich man, boasting of his exact performance of the law, whether, when he pretended to aspire to eternal life, he would aspire to that perfection which his countrymen so praised. Not that hence he either devoted Christians to voluntary poverty, or that he exhorted this man to rest ultimately in a Pharisaical perfection; but lifting up his mind to the renouncing of worldly things, he provokes him to it by the very doctrine of the Pharisees which he professed.

"For these things the measure is not stated; for the corner of the field" to be left for the poor; "for the firstfruits for the appearance in the Temple" (according to the law, Exodus 23:15,17, where, what, or how great an oblation is to be brought, is not appointed), "for the shewing mercy, and for the study of the law." The casuists, discussing that point of 'shewing mercy,' do thus determine concerning it: "A stated measure is not indeed prescribed to the shewing of mercy, as to the affording poor men help with thy body," that is, with thy bodily labour; "but as to money there is a stated measure, namely, the fifth part of thy wealth; nor is any bound to give the poor above the fifth part of his estate, unless he does it out of extraordinary devotion." See Rambam upon the place, and the Jerusalem Gemara: where the example of R. Ishbab is produced, distributing all his goods to the poor.