What should have I done?

I was waiting for the train at the station today and a lad asked me if he could have 7p. I was taking 1p coins out of my purse to make up 7p and the lad said 'no, no' then he said �1, so thinking he wanted �1 in return for loose change, I gave him a pound then he asked for another pound. He took two pounds off me, said 'thank you' and blow me a kiss then walked off. It's not an ideal situation and I don't understand why I did what I did. All I thought was that he wanted some change but instead he took money from me. How could I have made myself more ertive?