John Lightfoot on Matthew 20:13

[Didst not thou agree with me for a penny?] In hiring of labourers, the custom of the place most prevailed; hence came that axiom, Observe the custom of the city; speaking of this very thing. There is also an example, "Those of Tiberias that went up to Bethmeon to be hired for labourers, were hired according to the custom of Bethmeon," &c. By the by also we may observe that which is said by the Babylonians in the place cited...as the Gloss renders it, "Notice must be taken whether they come from several places; for at some places they go to work sooner, and at some later."

Hence two things may be cleared in the parable before us: 1. Why they are said to be hired at such different hours; namely, therefore, because they are supposed to have come together from several places. 2. Why there was no certain agreement made with those that were hired at the third, sixth, and ninth hours, as with those that were hired early in the morning; but that he should only say, "Whatsoever is right I will give you": that is, supposing that they would submit to the custom of the place. But, indeed, when their wages were to be paid them, there is, by the favour of the lord of the vineyard, an equality made between those that were hired for some hours, and those that were hired for the whole day; and when these last murmured, they are answered from their own agreement, You agreed with me. Note here the canon; "The master of the family saith to his servant, 'Go, hire me labourers for fourpence': he goes and hires them for threepence; although their labour deserves fourpence, they shall not receive but three, because they bound themselves by agreement, and their complaint is against the servant."