Around the time he visited the Novigrad region, he got drunk and foolishly put up for grabs his employer's title to property in a gwent match against Gerd. When he lost, he tried to desperately get the documents back, smashing his bottle against Gerd until Gerd pulled a weapon out on the dwarf, scaring him off.
However, not wanting to show his face back home lest he get the documents back, he tried to find someone to help him out.
If Geralt helps him: He spotted Geralt and told him how a thief took the important documents and the witcher soon went looking for the "thief". However, Rostan was soon confronted by Geralt, who learned the truth from Gerd, and so Rostan confessed what had happened and how he desperately needed the documents back. Geralt eventually returned the documents to him so he could return home.