A Maintenance Controller reports he authorized a deferral of #1 display unit supply fan per MEL 21-24-2C; even after a Mechanic told him the wrong fan was installed and needed to be replaced on a CRJ-200 aircraft.

Narrative

Authorized deferral of the #1 display unit supply fan (MEL 21-24-2c) due to a pilot write-up that the display 'Cool' caution message posted in flight at FL340. Station Maintenance stated that the fan needed to be replaced. During subsequent questioning about the reason for deferral; the Mechanic stated that the wrong P/N (Part Number) fan was installed and that was the reason it needed to be replaced; he provided the correct P/N. This happened early in the morning and I must not have been paying close enough attention because it did not register that we could not defer the system with the wrong part installed. This fact was pointed out to me when I arrived for work the following night by the Shift Supervisor. The aircraft was stopped and the part replaced and the MEL cleared.After being in the business for 40 plus years; I may becoming a little complacent or in need of re-education (refresher) on some of the intricacies of the FARs. I find classroom education much more valuable than self study or CBT (Computer Based Training).I need; and I think that anybody that has been in the business very long; needs real Recurrent Training periodically; just to prevent issues like this from creeping into the daily operation. As stated before; self-study or CBT education is not [as] effective as the real thing.