Estoppel

Estoppel is the general legal term of art[?] that is used in the common law to refer to the doctrine that prevents a party from using his own acts to claim a right to the detriment[?] of another who was entitled to rely upon the conduct of the party and acts upon that reliance.

A simple example is the case where you go to a store and see a sign that the price of one of store owners product's a radio, is $10. You speak with the owner and tell him you will get the money and come back later that day to purchase it, there is no discussion of price. He says that when he returns he will be happy to deal with you as he deals with all his customers, but that if sells all the radios (he has three) then he will not be able to help you. You go sell your watch for $10 which was really worth $15 but since you wanted the money right away you could not wait for the best price and you sold it to someone who you knew would take $10. When you return the sign says $11, and the owner tells you he has changed the price. In equity he may be estopped from his conduct. You relied upon his representation that he would sell you the radio when you came back the same day with the money; you had sold your watch at a price lower than the market price and thus you have acted to your detriment. (Note if your watch was worth $10 and you received fair price there would not be any detriment on your part).

Note, in some common law jurisdictions if when you had approached the owner and said indicated that you wanted to purchase one of those radios and he had said, 'sold' you may be able to argue that the contract had been created then, even if you had to go get the money. But under the classical idea of consideration until you paid him the contract would not have been concluded.

Also sometimes referred to as deterimental reliance The concept of estoppel is applied in many areas of law.

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