When it comes to describing a small amount of money, there are many ways to say it without using the term "petty sum." Some synonyms for "petty sum" include nominal amount, trivial expense, a pittance, a token fee, a meager payment, and a minuscule sum. Other phrases that could be used include "a small chunk of change," "a paltry sum," "a small outlay," "a drop in the bucket," and "a modest sum." It's important to remember that the terms used in describing an amount of money can have an impact on the way a transaction is perceived, so it's important to use the right language for the occasion.