The small amount of extra money a business keeps on hand is known as petty cash. If you work at your aunt's hardware store, she may give you ten dollars out of petty cash to buy all the employees donuts.
Any little costs that come up, from buying stamps at the post office to tipping a delivery driver, can be covered using petty cash. In most small stores or offices, petty cash might be a hundred dollars but would rarely amount to more than that. While today we generally use petty to mean "unimportant" or "trivial," the phrase petty cash uses its original meaning, "small."