What is the ruling of Islamic law regarding a tenant's refusal to pay a portion of shared housing expenses, claiming to have become a "guest" due to his housemates hosting other guests for extended periods, and what is the ruling on the tenant's action in the second scenario, where he refused to pay his share of the rent and expenses to the person who replaced one of the original tenants, considering the latter's presence illegitimate because he entered the dwelling without his permission?

The questioner was not obligated to cover the expenses of the guests or participate in them, as they were not his guests, and because the obligatory hospitality does not exceed three days. What was spent with the intention of voluntary charity cannot be reclaimed, for the Prophet, peace and blessings be upon him, said: "The one who takes back his gift is like a dog that returns to its vomit." However, if it was spent with the intention of reclaiming it, he has the right to demand it back with proof. What the questioner spent was not with the intention of reclaiming it, so he is not entitled to demand it back. The rent for the accommodation and the expenses remain a debt upon him. His continued stay in the accommodation and his utilization of it obligates him to fulfill his financial obligations.