He chased them down to return the cash as they drove off.
Bismark Mensah, 32, worked at the Walmart in Federal Way, Wash. and was helping Leona Wisdom and Gary Elton load their items.
It was only after the couple drove off that he discovered an envelope full of cash was left in their cart. He could tell it was cash because the envelope had a small window on the front.
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Without even thinking twice, Mensah chased down their car and got them to stop, handing Wisdom the money.
"I run after them. I think somebody heard me and signaled for them to stop," he said.
"She was like, 'Wow!' Tears are coming out. She took some money and tried to reward me. I said, 'No, no. I'm all right.'"
The couple was planning to use the money for a downpayment on a house they were purchasing, and they did not want to wait for a check to clear so they were going to use cash.
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Mensah only made around $620 to $640 a week at the time, and worked mainly in the parking lot and in inventory. He desperately needed more money, but said he would not allow himself to take something that wasn't his.
"My conscience wouldn't allow it," he said. "I couldn't even drive home if I did that."
Mensah was awarded with the "Integrity in Action Award" from Walmart for returning the cash to the couple. He was also promoted to a full-time job after he started working there part-time when he emigrated from Ghana in 2012.
Wisdom said she called Walmart two times to make sure management knew what Mensah had done. She tried to think of ways to thank him, but he declined all of them.
She even tried to hook Mensah up with her single daughter, asking him if he was single.
"It's hard to find honest people," she said.
But he denied that offer as well.
It is not the first time management has received a call about Mensah, as store manager Jeremy Smith said customers regularly call them to tell them how helpful he was.
"Maybe they were trying to load something heavy into their vehicle. He rushed right away to help them. They were overwhelmed with his kindness and generosity," Smith said.