"A woman came up and said: 'Excuse me, but you look like really nice people. Can I borrow some money? We forgot our wallets," sisters Mimi and Titi Negussie told The Sun newspaper.
The pair, who didn't even recognise Madonna, coughed up a £2 coin to cover the extra in the £5.25 bill.
The sisters will be sent a cheque and CDs to say thank you, a spokesman for Madonna said.