The Charity Commission is warning smaller organisations to watch out for the scam, after the Brian Byrnes Memorial Fund, a charity that supports Bangladeshi orphans, received a cheque for £4,500. Before it cashed the cheque the donor asked for £2,500 to be returned to another bank account, citing an "administrative error" for the overpayment.
Peter Byrnes, chair of the charity's trustees, became suspicious when he saw the email address of the donor was an internet cafe in Lagos, Nigeria.
"It was so believable - the cheque was from a genuine bank and I spoke to the donor on the phone several times.
"We're used to spotting scams, but because we received a cheque from an individual, we took it to be legitimate. I'm just relieved that we didn't make any repayment, as the cheque bounced."
In another case, Bullying Online was offered £10,000 by bank draft, but director Liz Carnell became suspicious at the size of the donation, and refused a request to pay part of the money into another account.
Simon Gillespie, director of operations at the Charity Commission, urged charities never to return any donations before the cheque has cleared.
He said: "This is a very sinister operation because it is so plausible.
"These requests should be treated with great caution. If trustees have any suspicions they should contact the police or call the commission's helpline on 0870 333 0123."
With so many charities existing on an income of less than £10,000, this kind of fraud could be devastating, the commission warned.