Does the increase in the value of a travel ticket in hard currency after its conversion to the national currency revert to the employee or to the employer?

The employee is entrusted with the work and money committed to them. Any increase in the entity's funds belongs to its owners, not to the employee. The employee acts as an agent for the employer, and therefore must hand over all proceeds from sales and purchases to the owners of the money. Thus, any increase belongs to the principal, not to the agent.