Shaykh Abdur-Rahman ibn Nasir al-Sa‘di (rahimahullah):

Regarding the person who is entrusted with wealth by the consent of its owner or is in charge of it; including someone who holds wealth for safekeeping, a trustee (who acts as a representative of another), a renter, a business partner, etc. If the wealth or thing they have in their possession is damaged, lost or stolen without negligence or transgression on their part, then they are not liable. If this occurs while in their hands then it is akin to it occuring while in the hands of the actual owner. However, if they transgress or are negligent, then they are liable. Negligence is defined as not safeguarding it as it deserves, while transgression is to deal with it or use it in a way that is not permitted.

(Slighty summarised from al-Qawaid wal-Usul al-Jami‘ah of al-Sa‘di, 14^th Principle)

TN — Example of this Principle:

If a person asks a friend to safeguard some money and the friend takes reasonable measures to secure the money but is robbed or his home is burgled, the friend is not obligated to repay the money. However, if the friend was negligent, such as leaving the money in an unsafe place or openly carrying it in a dangerous area, he is accountable and must repay the money.

Another Example:

If a person allows a friend to drive his car for a week, and during that time the car breaks down through no fault of the friend, the friend is not responsible for the repair costs. However, if the friend was negligent or reckless in driving, causing the breakdown, then he must cover the repair expenses.