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The Gemora cites a braisa: If one says to his fellow, “Go and hire workers for me,” neither of them will transgress the prohibition of holding a worker’s wages overnight. The employer cannot be liable, for he did not hire him (and the employee is therefore not regarded as “his hired worker”). The agent cannot be liable, for he is not the one who owes the worker his wages.
The Bechor Shor (Yevamos 96b) uses our Gemora to answer a famous question. Chazal say that there is no reward for the observance of mitzvos in this world; the reward will be given in the World to Come. They ask: How can this be? Isn’t there a prohibition against an employer to delay the payment of wages to his worker? How can Hashem wait until the World to Come?
He answers that since the Jewish people were not commanded to observe the mitzvos directly from Hashem; rather, it was done through Moshe, the prohibition does not apply. Hashem did not directly instruct Klal Yisroel to perform the mitzvos, and Moshe is not the employer.
This answer will be sufficient for Klal Yisroel; however, it does not answer regarding Moshe himself. He was instructed directly from Hashem to observe the mitzvos? How can his reward be delayed?
One can answer based upon the Gemora in Sotah (13b) which states that Moshe is constantly serving the Holy One, Blessed be He, and since rental is paid only at the end, his reward does not come due until the World to Come.