What is the ruling on paying a percentage of a project's value to a sales representative of another company in exchange for them nominating it to carry out the installation, knowing that such a nomination usually does not occur except by paying this percentage?

If there is no deception or fraud in your sales representative's nomination, then there is no harm in you giving him money for it, provided that he is authorized by his employer to nominate whoever he deems fit. The contract in this case would be one of ju'ala (commission/reward). However, if he is not authorized by his employer to take a fee for it, then it is impermissible for him to request and take it, just as it is impermissible for you to give it.