Dear Readers, thank you very much for all your answers. You are all right from your respective points of view.
Indeed, it was a decision full of risk at a time when the man was dying of thirst. Nonetheless, he chose to follow the instructions on the note and poured the whole pitcher of water down the rusty old pump and fiercely pumped up and down. After few rounds of pumping up and down, the water gushed out of the pump, and the man was delighted to see that. He drank from it, and then filled the pitcher full and added a few words towards the end of the note which read, “This mechanism really works, you just got to trust and believe!”
Moral: Sometimes, it is very important to give away all that we have in order to get something back. And to do this we only need to have “conviction” that we will get something back.
Maybe the pitcher contained just the amount of water required for priming the pump. If the man had drunk a little water from it maybe it would have been insufficient for priming the pump thereby leaving the man die of thirst and no water for other travellers who would later reach the place in search of water.
No matter what, always remember “GIVING IS IMPORTANT !!!”