What is the ruling on salary and investment in a private company that acts as an intermediary for foreign companies, pays sums of money or gives gifts (sales development) to buyers, and deals with usurious loans, knowing that the employee had previously signed disbursement approvals for sales development?

If the money paid is a forbidden bribe, then it is not permissible to pay it, assist in it, or approve of its disbursement. One must repent from such an act. A forbidden bribe is that which is used to obtain something to which one has no right.

As for your work in a company that takes out forbidden usurious loans, if your work is in permissible areas and has no connection to these loans, then it is permissible, though leaving it would be better. However, investing in this company is not permissible, because an investor is a partner in it, and one who partners in a company that takes out usurious loans is a partner with it in sin.