Is giving the contractor a promise-to-sell document to the buyer, who then uses it to obtain an interest-based loan from the bank, considered cooperation in sin, and must he stop giving this document?

If the document is necessary or helpful in obtaining an interest-based loan (riba), then it is not permissible to give it to someone who is known or thought to be using it for that purpose. This is because it constitutes cooperation in sin and aggression, which is prohibited by the Almighty's saying: "And do not cooperate in sin and aggression." And whoever abandons something for the sake of Allah, Allah will compensate him with something better, as the Almighty says: "And whoever fears Allah - He will make for him a way out And will provide for him from where he does not expect." As for someone about whom you know that he will not use it for riba, or if you are ignorant of his situation, there is no harm in writing it for him.