Mostly this is a family chapter focusing on how the Rostov family reacts to their son’s letter, but it still follows the theme of perception. Implied is that Nikolai carefully crafted the letter for his family and though we don’t read the letter we can deduce what he has and has not told them (the purse incident, for example).
This scene mirrors Petya’s much later.
6000 rubles. that’s a lot even today.