Kind of like tit for tat.

A woman came in to exchange two bras for two new ones. Unfortunately, the store no longer made the old ones, and only stocked the new & improved forms. She became irate when, without a receipt, the associate could not make an even exchange - the woman could have paid sale price, then come in to try and get the new, full-price bra. When the woman still did not accept this answer, the clerk finally put it like this.

"Look, Ma'am, it's like if you walked into Best Buy with a Gameboy and said 'I'd like that Wii please'. It's not going to happen."