I did it something similar a few years ago, Tesco had a double the difference offer, so if you bought some shopping for £20, you could take receipt home and enter receipt number in on website and if in asda/morrisons it would cost £10 then tesco would give you double the difference (£20) back in the form of a downloadable a voucher. The stupid thing was you could then go round tesco and use the voucher to repeat the process, earning club card points at the same time. They had a similar deal for mispriced items, ie I picked up a microwave that has a shelf price of reduced to £20, but it went through at the till at normal price of £49 so I then went to customer services (after paying for it) and got £58 back in cash.
And as you would expect there where forums posting lists of items that where a lot cheaper at say asda than tesco and the latest 'miss priced' items and where they were seen.
It's no surprise that tesco cottoned on to people taking the pee and after a while they changed the policy.
