John Gill Commentary Deuteronomy 24:15

"in his day thou shalt give him his hire, neither shall the sun go down upon it; for he is poor, and setteth his heart upon it: lest he cry against thee unto Jehovah, and it be sin unto thee." — Deuteronomy 24:15 (ASV)

At his day you shall give [him] his hire
At the close of the day, when his work is done, the hire agreed for must be paid him; and, by the same rule, all such that were hired by the week, month, or year, were to have their wages paid them at the day their time was up:

neither shall the sun go down upon it ;
it was to be paid before sun setting, or at it; see (Leviticus 19:13) ;

for he [is] poor, and sets his heart upon it ;
being poor, he cannot wait any longer for the payment of it; his personal and family wants are such as require immediate payment; and besides, he has been eagerly expecting it, and earnestly desiring it, that he may satisfy the craving necessities of himself and family; and therefore it would be a great balk and disappointment to him to have his wages detained:

lest he cry against you to the Lord ;
having none to apply unto but him, who is the patron of the poor and needy, not being able to help himself, nor having interest in any to interpose on his behalf; and his cry, and the cry of his hire too, enter into the ears of the Lord of hosts, and is regarded by him, (James 5:4) ;

and it be sin unto you :
be imputed to him, the guilt charged on him, and punishment inflicted for it.