A Good Samaritan who found $500 hidden in some cheap op-shop place mats has been rewarded for finding the owner and reuniting her with her money after 20 years.

Jan Hansch could not believe her eyes when she unfolded the second hand place mats her sister had bought for $3 from a Frankston op-shop, and $500 and a pay envelope, addressed to M.D Fife, from two decades ago fell on the floor.

Even though the cash would have come in handy, Jan didn't even consider keeping it.

"I just couldn't because that's obviously someone's pay," she told Seven News reporter Christie Cooper.

"It's their money, not mine."

Instead, she enlisted the help of her sister, Clare Fisher, and the pair took out the phone book, found all the Fifes in Victoria and eventually came across the rightful owner, Margaret.

Now a 72-year-old pensioner, former Target worker Margaret had hidden her fortnight's pay back in 2003, and then completely forgotten where she had put it.

Margaret packed up her unused items into boxes before some of them were taken to the RSPCA op-shop in Frankston, where Jan's sister made the lucky purchase.

Margaret gave Jan and her sister each a $50 reward, and used the rest of the money to fix her broken heater.