I answered this on your other thread :) But here it is again.
Create the event to change gold +(insert amount)
I always create a text box message saying something generic like, "Received 200 gold"
After you do this click on control self switch and turn self switch A on. then on the professor create a brand new tab at the top. Under this tab click on the left side where it says self switch and change to A. this means that this part of the event won't run until the self switch A is on. Make sure you put the Professor's graphic on this tab as well or he will vanish. If you want you can insert a new message to show on this tab when you try to speak to him like, "I already gave money, now get away from me!" This message will continue every time you try to talk with him and you won't get any more money from him.