Is the money taken by the claimant from the employer without his knowledge, after the latter refused to pay for the car accident that occurred during work, obligatory to return, or is it his right?

It is not permissible to take from a person's money without their knowledge, unless you have a proven right over them which they deny, and you cannot obtain it except through this stratagem. Regarding the car accident issue, the matter refers back to the employment contract between you. If the employer is obligated to cover the car's repair costs and refuses to pay for the damage, and you cannot obtain your right except by taking from their money without their knowledge, then it is permissible for you to do so, to the extent of your right. However, if their responsibility for the damage is not proven, then it is not permissible for you to take anything from their money without their knowledge.