What is the ruling on outstanding invoices owed to suppliers who sell stolen or illicit electronic software licenses, and is it permissible to sell the remaining stock of these licenses?

If the questioner knows that the keys are stolen, then they and their عوض (replacement/compensation) are the right of their developers, and the seller is not entitled to their price because he sold a benefit belonging to someone else without their permission. It is not permissible for the questioner to sell them for the same reason, which is disposing of someone else's property without their permission, as stated in the Mejelle: "It is not permissible for anyone to dispose of another's property without his permission." If this field is rife with theft, then he must refrain from it and seek a permissible field, for the Prophet, peace and blessings be upon him, said: "O people, fear Allah and be moderate in seeking (livelihood), for no soul will die until it has received its full provision, even if it is slow in coming to it."