How can one get rid of suspicions related to money acquired through potentially illicit means, especially given the existence of fatwas indicating that charity alone is not sufficient to absolve one from using public funds, and a strong desire to avoid unlawful acts?

If a father uses a work vehicle for an unlawful purpose, this does not render the supplies he bought with it unlawful. Rather, he is obliged to return the rental value of what he benefited from the vehicle. There is no harm in using what the father brought with the said vehicle, and avoiding the use of such things is closer to obsessive doubts and extremism.