Is the contractor considered sinful and earning ill-gotten gains if he submits a price proposal for a construction project, having prior knowledge of the price lists of other contractors through the assistant project manager, and then wins the project after submitting a price similar to the quoted prices?

The disclosure by the assistant director of confidential bids and his informing you of them constitutes a betrayal of his employer and an injustice to other contractors. This act is forbidden, for Allah Almighty says: "O you who have believed, do not betray Allah and the Messenger or betray your trusts while you know." Your request for this from him is also forbidden, for Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." The principle in tenders is free competition. If your bid is chosen, you may benefit from what you earn from it, along with repentance and seeking forgiveness. You are not required to dispose of the earnings or pay anything to the other contractors.