In our culture, once you have entered into a contract, it is not renegotiable. You can’t add conditions after the event.
Here's an example. A customer wanted to buy a new fridge. As part of the deal he was offered a replacement if the one he purchased went wrong, rusted etc within the first six years. He paid $150 extra for this.
The fridge was delivered and that afternoon he found a letter in his mailbox. It was the invoice and a copy of the details of the special offer for which he had paid the $150. He looked inside. There he found a long list of the exceptions, which would not entitle him to the new fridge. These were so extensive as to make the $150 investment almost valueless.
When he ordered the fridge on the phone, he was never told there were any exclusions.
He rang the supplier. As predicted, he was told he could cancel the contract and have his money back. He refused. Instead, he insisted the supplier honour his side of the bargain, exactly according to their contract, and that the exclusions should be deleted. After some wrangling, the retailer gave in.
If you're the seller and you do this, in most cases you’ll probably get away with it because not many people know they can contest it. If you're the buyer, stand your ground, if you wish to.
The retailer missed a golden opportunity. When faced with a situation like this, give in gracefully and quickly (and even better, never renege on a contract in the first place). The customer is much more likely to tell people what a wonderful firm yours is, instead of bad mouthing you and warning people about your business practices.
A contract does not have to be in writing. The extra $150 paid was sufficient evidence of the agreement.