The gratuitous promise, or unilateral obligation, is a feature unique to Scots law. It arises from the lack of a compensation requirement for Scottish contracts.
Basically, a gratuitous promise is a one-sided contract, where one of the two parties (the promisor) gets nothing from the other party (the beneficiary). The commonest example in case law is that of the wealthy little old lady who writes a letter to the vicar promissing to contribute to the local church's new roof. That is a gratuitous promise, and if she dies before the money is paid, the vicar can make a claim on her estate.
Obviously, gratuitous promises can be very risky things. Scottish law is very specific about what constitutes such a contract and how it works.
- A gratuitous promise is irrevocable if not rejected outright by the beneficiary.
- The promisor must intend to be legally bound when making the promise.
Statements of hope or expectation do not count as gratuitous promises.
- The promisor must communicate the promise directly to the beneficiary.
A potential beneficiary who hears about a promise by accident, or from a third party, cannot hold the promisor liable. The promisor may use any medium to communicate the promise, including advertisement in the press.
- A gratuitous promise can be conditional
For instance, the promise of reward for the return of lost property is a conditional gratuitous promise (since you own the lost property, its return is not compensation for the reward.)
- A gratuitous promise must be documented in writing
Until the Requirements of Writing (Scotland) Act of 1995, gratuitous promises had to be proven by writ or oath. However, this strong and somewhat obscure provision was eliminated as part of a sweeping set of modernisations in Scots contract law.
Source: Scottish Business Law, Second Edition, by Moira McMillan and Sally McFarlane, 1996.
I am not a lawyer, and nothing in this write-up constitutes legal advice. The textbook I used is out of date, and the law changes all the time. If you need legal advice, contact a solicitor; the best this write-up can do is help you communicate with any legal advisor you hire.
I am not convinced that the American concept of promissory estoppel is the equivalent of a gratuitous promise. It seems to a way of patching up a defective contract, bending the law to let justice through. Drennam v Star Paving is certainly a case where the ordinary law of contract seems to have been suspended.
This is different from Scots law, where the gratuitous promise is simply another form of contract, truly interesting only to systems where compensation is required.