How can one atone for the sin of purchasing a car in installments through a bank, knowing that all installments have been paid in full, and the purpose of the purchase was to avoid the hardship of public transportation and to help the family?

Borrowing with interest is usury, one of the seven destructive sins, and it is not permissible except in cases of extreme necessity. You must repent sincerely to Allah for this usurious transaction. Allah says: "And those who, when they commit an immorality or wrong themselves [by sinning], remember Allah and ask forgiveness for their sins - and who can forgive sins except Allah? - and [who] do not knowingly persist in what they have done. Those - their reward is forgiveness from their Lord and gardens beneath which rivers flow, wherein they will abide eternally; and excellent is the reward of the [righteous] doers." The questioner should increase in seeking forgiveness and performing righteous deeds. The conditions of repentance are remorse, immediately refraining from the sin, and resolving never to return to it.