Nothing wrong with doing that, for Any club to get 3 players off Man Utd on Loan is very lucky, and its clear Fergie was helping his son out, hes not at Preston anymore so the "good will" goes with Darren. Its not unprofessional or rocket science, just business.
Darren had exactly the same arrangement with Wrexham when he was there as a player and manager, anytime things got a bit ropey money wise, Man Utd would turn up and play a friendly and ties were close, as soon as Darren left, Man Utd didnt bother with Wrexham.