(CNN) -- About 12 years ago near closing time, a man in a hoodie with his face almost covered came into the InterAsian Market and Deli on Nolensville Pike Road in Nashville, Tennessee.

Somboon Wu, who was working that night with his father, remembers the night was cold. He also remembers the man buying beer, then asking for cigarettes. And when Wu's father opened the cash register, he remembers the customer pulling out a handgun and demanding all the cash in the register.

Wu says his father, Keosavanh Xayarath, always said two things: "Stay calm" and "Money can be replaced."

So he gave the guy the cash. The store had been open for several years by that point and had never been robbed before. Wu said that robbery was the only time his father had been threatened with a gun.

Wu said other than a deep gratitude that no one was hurt, the incident didn't really change the family too much.

Fast forward to September 6, 2013. Two men approach the younger Wu at the market and explain they have something to leave for the owner. One man places a white envelope on the counter.

"He told me to make sure the owner got it because there was money inside," Wu said. "I slid the envelope back toward him and told him he'd have to come back when my dad comes in. I was wary of these two."

The two men retrieved the envelope and stepped outside, where Wu watched them talk to each other. Then they walked back in, dropped the envelope on the counter and walked away.

"I didn't know what it was, so I took it to our attorney," Wu said. "He was floored."

In addition to $400, inside the envelope was a handwritten note on yellow notepaper. It begins, "I am a drug addict."

The letter goes on to explain, with a few misspellings, that the money and letter are an attempt to make amends to the store owner for the 12-year-old robbery.

The family posted a photo of the letter to Instagram and Twitter with the caption, "To the anonymous person we want to tell you all is forgiven and thank you for the note. We don't care about the money. We are more inspired and touched by your act. We hope you find peace in life and prosperity. Best wishes!"

Wu said his father wished he would have recognized the man. When asked what the men looked like, Wu said he didn't want to go into too much detail out of respect for his wishes to remain anonymous. Xayarath also told Wu he wishes he would have known then what the robber was going through.

"We think he has a lot of courage to come back and say he's sorry," Wu said. "He may not realize how many lives he's touched. If he happens to read this and if he ever wants to talk, he is welcome back here."