I admire your guts for doing it, and I'll support any honest busker if I have some cash - there are some cracking good ones working the London Underground who put my own playing to shame. But I don't think I'd ever have the nerve to offer a note/bill and ask for change! Do people really do that? Is it discourteous, or is it perfectly reasonable to say 'I'll give you two, do you have change for a five'? All the best - Ian B, Oxford