Take a penny, leave a penny
Take a penny, leave a penny (sometimes Give a penny, take a penny or penny tray) refers to a type of tray, dish or cup often found in gas stations, convenience stores, and other small stores in North America, meant for convenience in cash transactions. The phrase should not be confused with 'Spend a penny' which is British slang for urination.
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 when dealing with amounts slightly less than others easier to work with; 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 with an advertisement on it, such as for cigarettes, with text advising the customer to take a penny or 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.