The gemara says that if one uses a shliach to hire a worker, the employer would not be in violation of b'al tin for not paying the worker on time. Furthermore, the gemara cites amoraim who would davkahire workers through an agent to avoid the issur just in case they would not be available to pay when the time came (tosafos) [why doesn't tosafos say just in case they forgot to pay? This implies that forgetting to pay would not be a severe issur that would warrant such a precaution].
The rashash asks why don't we say shlucho shel adam k'moso, as we always do to b mechayev the employer. the rshash explains that ein shliach l'dvar aveira doesn't apply since the ma'aseh aveira is not violatd by hiring the worker, rather it is violated by the employer himself when he fails to pay. just as one who has relations with the mother of a woman he was mekadesh through a shliach is chayev and we don't apply the concept of ein shliach l'dvar aveira just because it was set up through a shliach.
i think that the rashash is muchrach according to tosafos in kiddushin by me'illa - 42b (?) - who says that when shliach is shogeg we say yeish shliach l'dvar aveira because the sevara of divrei harav vdivrei hatalmid doesnt apply. here too it certainly doesnt apply since the shliach doesnt expect the employer to delay payment.
Rashi seems to understand that it is almost a gezeiras hakasuv that the worker is not called a 'sachir' of an employer unless he hired the worker himself.