Here I would like to disclaim that whatever I am going to elaborate on its basic point have already been told by Swamiji, iam just providing a scientific point to it.
in 1850 first law of thermodynamics was propounded which states that total energy of a system will always be remain considered. It is said that the secret of the universe can be found in this law.
Alsothe great scientist Nikola Tesla have said that if you want to know the secret of universe think in terms of energy , radiation and frequency.so what happens when you do a act of philanthropy or act of charity you actually radiating a positive radiation in and sending a positive signals to the universe and Swamiji says that what ever we give to the universe it will eventually be back to us in multiplied factors. I’m calling it universe some may call at nature samay call it got some Angel some profit awesome Jesus and there are endless name but the basic idea is that there exist a cosmic consciousness which is very bad at keeping debts .
But very crucial point in giving DAAN (sanskrit tans. for Charity) is that it should be given selflessly because when we attach a selfish or a negative motive or a negative Vibe (trending slang but quite apt here) then along with that material we get that negativity multiplied. One has to be pure while giving. How to keep your? You ask me? Well seriously I am not in a position to tell you however only thing I can tell you is that if your following Swamiji and have firm believe in him and his and has read his work then you are on the right path as I think I am.