The pair told a member of staff at the Alzheimer's Society shop on King Cross Street, Halifax, that they needed advice because their grandmother had developed the condition.
But while the charity worker looked for a pen to fill out a help form they stole £80, which had been collected from raffle ticket sales, from a cash box on the counter.
Gail Waddington, the branch's development manager, said: "I'd left the cash box out to count the money.
"It's about as low as you can get, stealing from a charity."
When she returned from getting a pen, the cash from the tin was lying on the floor near to the couple.
"I realised the man must have tried to put the money in his pocket but dropped it. My brain started whizzing about what to do. I was quite scared because I was on my own in the office and there were two of them," she said.
Gail then saw the man pick the money up and stuff it into his trouser pocket.
Gail said: "I said 'Look I think that's my money. Can I have it back?'"
But the man denied the theft and the pair rushed out of the office.
"We'd collected about £80. It might not seem like very much money but we're very strapped for funds and it was money people wanted to go to the Alzheimer's Society."
A police spokeswoman said that officers were called to the Alzheimer's Society shop at 12.30pm on Monday.
The man is described as being in his 30s, with close-shaved light brown hair and of medium build. He wore jeans, a black tracksuit top and a baseball hat. The woman was in her 20s, with tanned skin and her short, dark hair was tied in two pigtails. She wore a brown leather-look jacket and brown combat style trousers.
Anyone with information can call police on 01422 337059.