What is the ruling on breaking a part that was originally worn out during repair, and is it obligatory to inform the customer of this, and am I sinful if the supervisor tells him that the part is defective while I remain silent?

One must be truthful and inform the customer of the real situation; lying to them is not permissible. If the supervisor lies to the customer, the questioner must clarify the truth to prevent the loss of rights and the wrongful consumption of wealth, as the Prophet, peace and blessings be upon him, said: "Help your brother, whether he is an oppressor or oppressed." Remaining silent about the truth is like a mute devil.

A joint employee (the workshop owner) guarantees damage if it is caused by his action, whether intentional or unintentional. However, if the damage is due to a cause beyond his action (such as theft), he does not guarantee it unless there is negligence or dereliction of duty on his part.

A private employee (the questioner) does not guarantee what is damaged to the workshop owner unless there is negligence or dereliction of duty on his part; because the item is a trust in his hand, and the guarantee falls upon his employer.