Bava Metzia 68B

אמר אביי: כפועל בטל של אותה מלאכה דבטל מינה.

Abaye says: It means that he is paid as a laborer who is idle from that typical labor of his from which he is kept idle. In other words, he must receive the amount of money that an individual would be willing to accept to refrain from his current occupation and engage in an easier task.

וצריכא, דאי תנא חנוני: חנוני הוא דסגי ליה כפועל בטל, משום דלא נפיש טרחיה. אבל מעות ליקח בהן פירות, דנפיש טרחיה ⁠– אימא לא סגי ליה כפועל בטל.

The Talmud comments: And it is necessary for the tanna to teach us both halakhot, that of a storekeeper who was given produce to sell as well as one who was given money to buy the produce. As had he taught only the halakha of a storekeeper who receives produce to sell, I would say that it is specifically a storekeeper for whom it is enough to receive his wages as an idle laborer, because his toil is not great, as the produce is already prepared and he merely has to sell it. But in the case of one who was given money with which to acquire produce, whose toil is great, as he must find the produce in the market and bring it back to his store, I might say that it is not sufficient for him to be paid as an idle laborer.

ואי תנא מעות ליקח בהן פירות ⁠– הוה אמינא: התם הוא דבעי כפועל בטל, משום דנפיש טרחיה. אבל חנוני, דלא נפיש טרחיה ⁠– אימא סגי ליה במשהו בעלמא, דאפילו לא טבל עמו אלא בציר, ולא אכל עמו אלא גרוגרת אחת ⁠– זהו שכרו, צריכא.

And conversely, had the tanna taught only the halakha of one who receives money with which to acquire produce, I would say it is in the case there that he requires payment as an idle laborer, because his toil is great, but with regard to a storekeeper, whose toil is not great, I would say that any amount is enough for him; that, For example: even if the one providing the produce only immersed his bread in brine with the storekeeper, or only ate one dried fig with him, this is sufficient to count as his wages, i.e., providing the bit of brine or a fig is sufficient to account for the storekeeper’s labor. It was therefore necessary for this halakha to be stated with regard to both cases.