The Convention distinguishes between withdrawal and revocation of an offer. According to Art. 15(2)
(2) An offer, even if it is irrevocable, may be withdrawn if the withdrawal reaches the offeree before or at the same time as the offer.
In other words, an offer can only be withdrawn before it becomes effective. After becoming effective by reaching the offeree withdrawal is not possible anymore. Yet, the offer might be revocable.