P bought cocoa from D who charged in Nigerian Dollars, but on P’s request agreed to charge in British pounds. They later revoked this by letter. D then wrote a new letter stating that payment could be made in either currency for all current and future purchases. Pound devalued and when P tried to buy in pounds (taking the letter to mean “the price is 12, so give us either $12 or £12), they were refused. HL ruled that the letter sent to P (paraphrased above) actually referred to payment of the purchase price in sterling and not its measurement in sterling. HL also said that the promise on which one can base a promissory stoppel defence has to be “clear and unequivocal, i.e., so expressed that, farfetched or strained interpretations apart, it would be understood in the sense required”. The letter was not clear enough.
Lord Hailsham: justice demands that promissory estoppel be clear so that the promisor does not bear any cost of ambiguity.