Article 16 CISG
(1) Until a contract is concluded an offer may be revoked if the revocation reaches the offeree before he has dispatched an acceptance.
(2) However, an offer cannot be revoked:
(a) if it indicates, whether by stating a fixed time for acceptance or otherwise, that it is irrevocable; or
(b) if it was reasonable for the offeree to rely on the offer as being irrevocable and the offeree has acted in reliance on the offer.