When he opened the letter paper, a three-rouble bill landed gently on his leg, forming the end of the word 100.

This is a complete story in itself, with good expressive force, profound implication and clear characterization. If you don't have a deep literary foundation, don't add it.

If you really want to add it. Give you some instructions.

1.3 rubles is the fare. Mother passed away. Mother gave him money for the winter.