What is the ruling of Islamic law on dealing with a payment intermediary who refuses to deliver the full amount to the company and instead delivers it in installments, knowing that the intermediary was chosen by the company and not by the client?

The one entrusted with a matter must carry it out in a way that serves the interest of the one who entrusted it, for he is given a trust. Treachery is forbidden by Islamic law and is a sign of hypocrisy. If what this person desires contradicts the interest of the company, then he is not permitted to do it except with the company's permission. You must forbid him from doing that and not assist him.