I am soon to visit a chasidic rebbe for a one-on-one meeting, for almost the first time in my life. I understand it is both appropriate and customary to (if feasible) leave "pidyon" — that is, money or a check made payable to the rebbe's congregation or organization. Do I give the check to the rebbe? give it to his gabay (secretary/gatekeeper)? leave it on the rebbe's desk? or what? Do I do so before (or at the start of) my meeting? after (or at the end)? or when? Any further details will be appreciated also.