Why did the banker lock up the young man's writing in the fireproof safe?

In essence, we aren't informed why the banker locked the young man's writing in his safe. Possibly, he meant to read it later. Maybe, he didn't want anyone else to see it... though he could have burned it if that were his intent. Likely, he simply wanted a record of the bet and the fact the young man had renounced his winnings.

The banker went at once with the servants to the lodge and made sure of the flight of his prisoner. To avoid arousing unnecessary talk, he took from the table the writing in which the millions were renounced, and when he got home locked it up in the fireproof safe.