Am I entitled to take what shop owners offer me as "gasoline money" after purchasing from them, knowing that my employer does not give me enough for my work and deducts from my salary without just cause?

Firstly, it is not permissible for an employer to deduct from an employee's salary without a legitimate religious reason (masūgh sharʿī), and the worker has the right to demand their due through legitimate means. Secondly, it is not permissible for an employee to accept gifts or gratuities offered to them because of their work, such as discounted prices on goods, free samples, or small sums of money. These must be returned to the shop owner unless he permits the employee to take them, because such gifts may lead to the employee compromising their trustworthiness or the business's interest. Thirdly, if an employee accepts any of these gifts, they are obligated to return them to their owners. If they are unable to do so, they must hand them over to the entity or institution on account of which the gift was given.