Bava Metzia 92A

ומהלך כעושה מעשה דמי.

but a worker who was walking is considered like one who was performing his labor. Yet, since one who was performing labor on this vine may not eat from another vine, he would not be entitled to eat while walking if not for the ordinance of the rabbis.

ובחמור כשהיא פורקת. כשהיא פורקת מהיכן אכלה! אימא עד שתהא פורקת. תנינא להא, דתנו רבנן: חמור וגמל אוכלים ממשאוי שעל גביהן, ובלבד שלא יטול בידו ויאכילם.

§ The Mishnah teaches: And with regard to a donkey, it is permitted to eat when it is being unloaded. The Talmud is puzzled by this statement: From where can it eat when it is being unloaded? Since the load is being removed from the animal at the time, how can the donkey eat from it? Rather, you should say: It may eat until it is unloaded. As long as it is bearing its load the donkey may eat from the food on its back. The Talmud comments: We learn in the Mishnah that which A baraita states explicitly in a baraita: A donkey and a camel may eat from the load on their backs, provided that the owner of the animal does not take some of the food in his hand and feed them.

מתני׳ אוכל פועל קישות אפילו בדינר, כותבת ואפילו בדינר. רבי אלעזר חסמא אומר: לא יאכל פועל יתר על שכרו, וחכמים מתירין. אבל מלמדין את האדם שלא יהא רעבתן ויהא סותם את הפתח בפניו.

Mishnah: A laborer may eat cucumbers while he works, and this is the halakha even if the amount he eats is equal in value to a dinar; or he may eat dates, and this is the halakha even if the amount he eats is equal in value to a dinar. R' Elazar Ḥisma says: A laborer may not eat more than the value of his wages, but the Rabbis permit it, according to the strict letter of the law. But one teaches a person not to be a glutton and thereby close the opening to other job offers in his face. When people hear of his greed they will be reluctant to hire him.