The Oscar-nominee was approached in Camden, north London by the strangers, who offered to clean ice cream smears from his jacket - but when he took the garment off, they swiped the cash out of his pocket.
Writing in The London Review of Books, Bennett explains, "They very kindly helped me to clean it up with tissues from one of their handbags and another man, English, I think, big and in his fifties, goes away and comes back with more tissues.
"The ice-cream (coffee-flavoured) seems to have got everywhere and they keep finding fresh smears of it so that I take my jacket off to to clean it up. No more being found, I put my jacket on again, thanking the women profusely, though they brush off my gratitude and abruptly disappear.
"I go back to the car, thinking how good it is that there are still people who, though total strangers, can be so selflessly helpful. It's only when I'm about to get into the car that I remember the money, look in my inside pocket to find, of course, that the envelope has gone."
The writer reported the incident to police but admits it affected his faith in people: "The casualty, though, is trust, so that I am now less ready to believe in the kindness of strangers."