Is there sin involved or is there an obligation to pay the cost of repairs for a consulting engineer who used a contracting company's car for personal purposes, and it was involved in an accident that resulted in its total damage, with the company covering the full repair costs?

If a borrowed car is damaged without negligence or transgression on the part of the borrower, then the borrower is not liable for it. However, if the borrower is negligent or transgresses, then he is liable for it. If the owning company pardons him, his liability is waived. It is important to note that the company's pardon might be a bribe to the borrower, in his capacity as a consulting engineer, to favor the company. In this case, neither borrowing the car nor accepting the pardon is permissible. If the lender is not authorized by the car's owner, then the borrower is liable for the car.