Maximilian Borsodi's house is a quest item in the Hearts of Stone expansion.
It's a miniature model mansion that serves as a special box for Maximilian Borsodi's will and property ownership. In particular, the will contains a clause where both of Maximilian's sons, Horst and Ewald, must meet once a year to retain family ownership of the Borsody Auction House, otherwise the property will be forfeited and auctioned off.
Olgierd von Everec somehow learned about this clause and, knowing the Borsodi brothers had failed to honor it, wished to get a hold of the documents to strip the Borsodis of everything.
Near the end of Open Sesame!, regardless of which Borsodi brother is still alive, they'll refuse to hand it over to Geralt. They can either be killed outright to obtain the box and documents within or reach a compromise, in which case only the box is handed over while the documents remain with the living brother.