Horst Borsodi, owner of the most famous auction house in all the North, was a snob and a buffoon. He looked down his nose at Geralt from the very moment they met, and when he heard what had brought the witcher to him, he had his men toss him out and beat him. To say their relationship got started off on the wrong foot would thus be putting it very lightly indeed...
Nevertheless, when the Borsodi brothers faced off in a final showdown, Geralt chose to stand by Horst. He never revealed why he made this decision – perhaps he considered a pompous buffoon less of an evil than a bloodthirsty cheat, which is what his erstwhile partner, Ewald, had proven to be. With Geralt's support, Horst managed to defeat Ewald.
...but also demands the papers:
When Horst revealed that, despite his earlier promises, he did not intend to give Geralt Maximilian's House, the witcher decided to take it from him by force. Horst died by the witcher's blade before his hated brother's corpse had even cooled.
...but does not take the papers:
He repaid the witcher for this by giving him the ornamented box known as Maximilian's House.
It came as no surprise, then, when Geralt chose to stand by his partner, Ewald, in the final showdown between the two brothers. With their strength combined, they defeated Horst's hired muscle. A few moments later, Ewald killed his hated brother in a spectacularly cruel fashion. It was later said none walked behind the once rich and mighty Horst Borsodi's casket and none shed a tear over his grave...