Answered by Shaykh Amjad Rasheed
I live in the university dorm with a Muslim brother. Do we have to ask one another for permission for eating food belonging to the other that is in the refrigerator? What are the boundaries of the permissibility?
The guiding principle in our school for using the belongings of others is as follows:
1. If the owner gives one permission to use what he owns; or if one is confident that the owner will give his consent, then it is permissible to use it.
2. Otherwise it is not permissible.
3. If one doubts that the owner will give his consent, it is not permissible to use his belongings.
It is obvious that one may consent to another using some of one’s belongings and not others. It is obligatory for the one wanting to use it to infer that from the state of its owner, because the default is unlawfulness of its use.