Every line of what you've written here is true. We all have a way we can affect other people's life, being able to do those things with the right mindset is generosity.

Generosity is an investment for eternity. We are to use our worldly resources for eternal gains. We sow ahead...we water ahead...

There are rewards for every act, both on earth and in eternity, and like you said, sometimes we reap in hundreds of folds, not measureable to what we sowed. That's why the Bible encourage that we shouldn't be weary in well doing because in due time we will receive our reward.