Art. 90 CISG
This Convention does not prevail over any international agreement which has already been or may be entered into and which contains provisions concerning the matters governed by this Convention, provided that the parties have their places of business in States parties to such agreement.
Overview of case law on Art. 90 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.