Pre-nuptual agreements were made by notary publics (notariusz) and are kept
in regional archives in Poland. Each notary kept his own books, and they are
not indexed. The fact that you have the date may be helpful. You can write the
main archives in Warsaw [Naczelna Dyrekcja Archiwow Panstwowych, ul. Dluga 6,
skr. poczt. 1005, 00-950 Warsaw, Poland] and ask if notary books for that
period exist and ask where they are located (probably either AGAD or Archive of
the city of Warsaw.) You would need to hire a researcher to look through them.
If the name of the notary is mentioned (is that the Anton Kokhanowski?), that
would make it much much easier to find.