Is the tenant entitled to claim the costs of essential improvements and modifications he made to the dwelling with the landlord's consent, and how are these costs estimated after twenty years, is it at the exchange rate at the time of expenditure or at today's rate?

An unspecified-term lease is invalid, and the tenant is not entitled to key money. As for the renovations and repairs carried out by the tenant in the property, the tenant can claim them from the landlord if they were for the benefit of the property and were done with the landlord's permission. However, if they were solely for the tenant's benefit, the tenant cannot claim them unless this was stipulated. The tenant can claim from the landlord the amount spent on the renovation and repair on the day they were carried out.