Bava Metzia 73A

ברשות מוכר ⁠– מותר, ברשות לוקח ⁠– אסור.

then, if the package remains in the possession of the seller, i.e., the seller accepts upon himself responsibility for any accidental damage that occurs along the way, it is permitted, as the transaction is not a loan. But if it is in the possession of the buyer, meaning that the buyer accepts responsibility for accidental damage, then the transaction is prohibited, as it is considered a loan with interest.

המוליך פירות ממקום למקום, מצאו חבירו ואמר לו: תנם לי ואני אעלה לך פירות שיש לי באותו מקום, אם יש לו פירות באותו מקום ⁠– מותר, ואם לאו ⁠– אסור. והחמרין מעלים במקום היוקר כבמקום הזול, ואינן חוששין.

With regard to one who transports produce from one place to another place, if another finds him and says to him: Give the produce to me now and I will repay you with produce that I have in that place to which you are going, then, if he actually has produce in that place, it is permitted, but if not, it is prohibited. But donkey drivers who transport merchandise from one place to another may accept money and set prices in a place where goods are sold at expensive prices according to the rate in effect in another place, where goods are sold at inexpensive prices, and need not be concerned, as this practice is permitted.

מאי טעמא? רב פפא אמר: ניחא להו דמגלו להו תרעא. רב אחא בריה דרב איקא אמר: ניחא להו דמוזלי גבייהו.

The Talmud asks: What is the reason this is permitted? Rav Pappa says: It is satisfactory to them to sell merchandise at a discounted rate, because by doing so the gates to the new market are opened for them, as in this way they begin to do business in this area and gain new customers. Rav Aḥa, son of Rav Ika, said: It is satisfactory to them because the prices are reduced for them in the places where they make their purchases. Since the sellers there hear that the donkey drivers will need to resell the merchandise at a lower price, the sellers give a discount to the donkey drivers. According to either opinion, the donkey drivers provide the additional produce to the customer not as interest on the loan but as a discount to promote their business.