Art. 51 CISG
(1) If the seller delivers only a part of the goods or if only a part of the goods delivered is in conformity with the contract, articles 46 to 50 apply in respect of the part which is missing or which does not conform.
(2) The buyer may declare the contract avoided in its entirety only if the failure to make delivery completely or in conformity with the contract amounts to a fundamental breach of the contract.
The Secretariat Commentary on Art. 47 of the 1978 New York Draft, predecessor to Art. 51 CISG, gives insight into the drafters' unterstanding on the eve of the 1980 Vienna Diplomatic Conference.
Overview of case law on Art. 51 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.