Shaykh Bin Baz was asked:
“If an individual borrows things and doesn’t return it at its specified time, or he doesn’t return it and is easygoing regarding it while the owners of it are in need of it, is there a sin upon him in that?”
“If it was for a specified time, then it is obligatory that he returns it after the time has finished, and if not, [i.e. if he does not do that] he is sinning. For example, if they give him the car for a day or two and he returns it after this time period [he is sinning]. Another example is they give him different needs which he benefits from and he returns it after its specified time [and so in this case he is also sinning]. It is not permissible to be easygoing regarding this because it is oppression. If he is easygoing, it is oppression and from the means of corrupting the affairs between neighbours and companions. If he transgresses upon the borrowed items [by breaking or losing it, etc], he is liable.
The intent is that he should pay concern to that which he has borrowed and that he protects it, that he returns it at its specified time. If there is no specified time, if he has completed his need, he rushes to returning it.” End. [Reference]
Majid Jawed Al-Afghanee