What kind of proof did the man probably have when he bought the radio?

M: Good morning, Miss. I’d like to speak to the manager.
W: I am the manager, Sir. How can I help you?
M: Oh, really? It’s this radio. It doesn’t work.
W: Mm... did you buy it here?
M: What do you mean? Of course I bought it here. Look, you switch it on and nothing happens.
W: Could I see your receipt?
M: Receipt? I haven’t got one.
W: Oh, you should have got a receipt when you bought it.
M: I perhaps did. I must have thrown it away.
W: Ah, well, have you got any other proof of purchase, the guarantee, for example?
M: No. It must have been in the box. I threw that away, too.
W: Oh, dear. You really ought to have kept it. We need to know the exact date of purchase.
M: What? I only bought it yesterday! That young man over there served me. Oh, I paid by cheque. I’ve got the cheque stub.
W: That’s all right then. Did you check the radio before you left the shop?
M: Check it? No, it was in the box. I hoped it would work. It wasn’t a cheap radio. It’s a good make.
W: You should have checked it.
M: Come on! Do not tell me what I should have done, and do something! Either give me my money back or give me another raido.
W: There’s no need to get aggressive, Sir. Let me look at it... mm... you see this little switch on the back?
M: Yes?
W: It’s on "main", and it should be on "battery". You really should have read the instructions.
M: Oh!