Is working at a passenger transport company that involves balancing the treasury and transferring money between accounts to avoid usurious interest, and contacting an investment company to deal with financial surplus—with the possibility that the investment might be in prohibited activities—permissible or forbidden?

If it is highly probable that the company uses the surplus in unlawful activities, such as usury, then it is not permissible to contact them to inform them of the surplus or to deliver it to them. This is because it constitutes aiding in sin, as Allah Almighty says: "And do not cooperate in sin and aggression." However, if the matter is merely a doubt, you are not obligated to investigate. If you can avoid informing the company, then there is no harm in continuing with the job. Otherwise, it is not permissible to remain in it except out of necessity and the absence of an alternative.