Narcissists can be very generous, but usually to display how kind and thoughtful they are, i.e. gain a good reputation which is a status seeking behavior. Example, if a narcisscist wants to leave someone an extra big tip, they'll make sure others will notice or draw attention to it "well, you have made my day, you earned this, go buy yourself something nice!!" Rather than just quietly handing it to them or stuffing it in the jar

Edit: another prime example is people recording themselves giving free stuff to homeless people, instead of, just giving it to them and not recording proof to plaster it all over the internet for asspats