A woman goes into a jewellers' shop and buys a gold ring for £100 (which she pays for in crisp, £20 notes). As she is leaving the shop, she pauses, and then goes back to the counter. I've decided I don't want this £100 ring - I want that £200 ring instead, she says. The jeweller takes back the £100 ring and gives her the £200 one. That will be another £100 then, madam. At which point the customer stamps her foot and says: I don't owe you anything. Earlier on I gave you £100 in cash for the ring. And now I've just handed over the ring as well, which is worth £100. So I've given you £200, and you have given me a £200 ring in return, so we are quits. And with that, she walks out of the shop, leaving the jeweller scratching his head.

Question: can you work out what has gone wrong?