Strike a Bargain
(To). In Latin, foedus ferire; in Greek, horkia temein. The allusion is to the Greek and Roman custom of making sacrifice in concluding an agreement or bargain. After calling the gods to witness, they struck —i.e. slew - the victim which was offered in sacrifice. The modern English custom is simply to strike or shake hands.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894