Hi Rhapsody89, and welcome here!
Now I want to pour a little bit oil in a fire.
If you have already opened your new treasure and have examined its guts ( i.e. what is inside ), you should have found out that the thing has got an electric motor. it is used in Hammond to rotate so called tone wheels, with the means of which the sound will be actually produced.
So, to cut the story short - this motor would need oil sometimes, like any other motor. The contact pairs ( not sure, but I believe there must be about 17 contact pairs per key by a B3 modification ) should be kept clean. In other words, the thing would need a certain maintenance, if you want to play it yourself , and if you want that it sounds properly.
Donating a Hammond to anybody is generally a brilliant idea, and your Grandpa would love it too. Man, why am I not your neighbour??? Can you somehow ship it to Germany???
Btw does this Hammond come alone, or with one another strange looking thing ( looks like a big wooden case; very heavy )? If yes - much respect to your Grandpa, because this thing - if you have it - is called Leslie. It contains an electric motor, a pair of rotating speakers, and an already mentioned wooden case. Can be sold OR DONATED together with the Hammond, or separately.
Once again - how about a shipment to Germany?
Truly Yours,
Andrew