The offer's revocability
In French law it is a principle that an offer is revocable (Cass. civ. 1ère, 3 February 1919). Unless the offer expressly contains a period within it has to be accepted, the offeror has the obligation to keep the offer open during that period.
In German law on the ground of the concept of zugehen, when you are the offeree it is sufficient (i.e. a letter containing an offer delivered at your house). The offer can be withdrawn until that moment and after that it becomes irrevocable (RG, 25 October 1917). The presence or absence of the offeree at the reaching of the offer is not important (§ 130 BGB).
In English law, an offer is revocable on the basis of the doctrine of consideration (Dickinson v Dodds) except it is a case of a unilateral contract (Daulia v Four Millbank Nominees). According to this concept the offeror is free to revoke the offer until the offeree has not performed the condition.
So it is interesting to observe that the French and the English law are reach the same result but through different legal concept. Indeed an offeror is free to revoke his offer if no condition or limit is adjunct to it. While in the German law system an offeror has to be much careful by making an offer than in our both aforesaid legal system because it will be for him impossible to revoke it. The Geman point of view is also different than article 2:202 (1) of the PECL and article 2.1.4 subsection 1 and article 2.1.4 subsection 2 of the Unidroit Principles which are much alike the French legal system.
Pierre-Patrice TIFI MAMBI