This is a kind and generous thing to do, but can only be accomplished if you tell them what you are doing. In order to pay them, you will need to send them a hire offer and have them accept it. You can then pay them a bonus in any amount, notating it with any reason you care to write, and then you can close the contract, marking it as completed successfully.
This will simply mean that your getting advice and giving them a bonus was completed successfully. This will not be an assessment of the never-started project.