Art. 50 CISG
If the goods do not conform with the contract and whether or not the price has already been paid, the buyer may reduce the price in the same proportion as the value that the goods actually delivered had at the time of the delivery bears to the value that conforming goods would have had at that time. However, if the seller remedies any failure to perform his obligations in accordance with article 37 or article 48 or if the buyer refuses to accept performance by the seller in accordance with those articles, the buyer may not reduce the price.
The Secretariat Commentary on Art. 46 of the 1978 New York Draft, predecessor to Art. 50 CISG, gives insight into the drafters' unterstanding on the eve of the 1980 Vienna Diplomatic Conference.
Overview of case law on Art. 50 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.