This sort of thing used to happen in the Bank I used to work in. The answer is to keep the money they give you for the first transaction on the top of the till, then when they start asking for further change. Say 'hang on, hang on', so you gave me �30 (or whatever), I gave you that in �5 notes, now you want �20 notes (or whatever), lets start again, give me back the money I gave you, I'll give you back yours and we'll start again. Stops them every time! Don't worry yourself too much about it, it happens in loads of shops, it was always happening to my father-in-law in his newsagents. Just don't let yourself be conned a second time.