Art. 43 CISG
(1) The buyer loses the right to rely on the provisions of article 41 or article 42 if he does not give notice to the seller specifying the nature of the right or claim of the third party within a reasonable time after he has become aware or ought to have become aware of the right or claim.
(2) The seller is not entitled to rely on the provisions of the preceding paragraph if he knew of the right or claim of the third party and the nature of it.
The Secretariat Commentary does not adress Article 43 CISG, since there was no equivalent provision in the 1978 New York Draft, which formed the basis for the Secretariat Commentary.
Overview of case law on Art. 43 CISG: UNCITRAL Digest
The UNCITRAL Digest on the CISG can be used as a starting point to discover the relevant case law on this article.