What is the ruling on selling a pump to a maintenance company after obtaining it for free, and what should I do with the money if the sale is invalid?

This money is rightfully owned by the questioner's employer in the public sector, not by him, because he acts as an agent for his employer in dealings with companies. In such contracts, a gift belongs to the principal, not the agent, as explicitly stated by Al-Bahuti. Even gifts not related to buying or selling are impermissible for an employee to accept without the permission of his employer, so long as they were given to him due to his position, based on the Prophet, peace and blessings be upon him, who said: "Gifts to officials are ill-gotten gains (ghulool)."