What can a person do with money acquired from a previous job that involved illicit overpricing, is it permissible for him to marry with it, and what is the best course of action to discharge the rights of thousands of people related to this money?

What has been mentioned about the company's increase over the stipulated cost for the cars is an injustice and an unlawful appropriation of wealth. You have erred by collaborating with the unjust company and by taking a portion of this increase for your own benefit. For repentance, you must dispose of this amount that was taken unjustly. Since its rightful owners are unknown, it must be spent with the intention of charity on their behalf. It is not permissible to use it for marriage as long as you are not in need of it.