Take a penny, leave a penny
Take a penny, leave a penny (sometimes Give a penny, take a penny, penny tray, or penny pool) refers to a type of tray, dish or cup meant for convenience in cash transactions. They are often found in the United States in gas stations, convenience stores, and other small stores, and were similarly common in Canada before the penny went out of circulation in 2013.
The small cup or tray near a cash register is designated as a place for people to place pennies they receive as change if they do not want them. Then, customers who, for example, need one cent for a transaction can take one of the pennies to avoid needing one of their own or breaking a higher-denomination coin or bill. The tray can also be used by cashiers for the sake of efficiency: the cashier may take a penny (1 cent) from the tray and then give the customer, for example, one quarter (25 cents) instead of 24 cents (two dimes and four pennies, or six coins in all). These are also called "penny pools", and may be either a generic container such as an ashtray, or a purposely made container, perhaps with an advertisement or with text advising the customer to "take a penny, leave a penny".
It is not uncommon to occasionally see nickels, dimes, and even quarters in a penny dish; a cashier short on pennies but nearing the end of his or her shift may swap one of the higher denomination coins for the equivalent number of pennies, especially if a large number of pennies are present in the dish.