In the town of Kelm, home of the famous Talmud Torah, which was the seed bed of the Mussar movement, there was a small general store for the use of the yeshiva students. In order not to take anyone from his studies to tend the counter, the store operated on the honor system; customers took what they needed and left payment in a cash box. After a while, it was found that there were overpayments in the box. Hearing of this, the Alter (Elder), R’ Simcha Zissel, ordered that the store be closed.
He explained. The reason there was too much money was probably because when the box did not have sufficient small change for customers, they would leave more money as a contribution. But by the same token, it was possible that people accustomed not to pay the exact amount might one day find themselves with too little money for a purchase, and they would feel entitled to underpay, because they had sometimes overpaid. This would be dishonest, and a situation that might encourage such a practice could not be continued. ~ The Pirkei Avos Treasury