From Law wiki, the wiki for law research

This case (Lampleigh v Braithwaite (1615) Hob 105) introduced the doctrine of Implicit assumpsit, which states that although it is a strong principle of English law that a past benefit cannot be invoked as Consideration to support a future Contract, this principle may not be operative when the past benefit accrued at the beneficiary's request, and with an understanding given that the benefit would be reward in the future.

Braithwaite was convicted of an unlawful killing. In desperation he asked Lampleigh to petition the King for a pardon. This was successful, and Braithwaite offered Lampleigh £100. However, when Lampleigh tried to claim, Braitwaite refused. His defence claimed Lampleigh had offered no consideration, so the agreement was not binding. The court ruled that in cases of this sort, where a past benefit was at the request of the beneficiary and where reward was expected, the past consideration could be 'assumed' into the agreement.