There are separate issues here.
1) If someone had the intention to give $100 to one person, then had a second thought to spread it around – several tzaddikim experienced this and felt this was “atzas hayetzer”. They followed their original plan. This experience is common, if we choose to examine our thoughts carefully. I would personally hate to allow my yetzer horah to influence my performance of mitzvos.
2) There is actually a beautiful discussion by the Mahara”l (Gur Aryeh – beginning of Parshas Trumah) where he notes that there were 3 trumos, of which two were each a Machatzis Hashekel, but the third was based on “Yidvenu libo”, according to the generosity of his heart. He explains that this was proportional, not based on the shekel amount. One who had less but gave more (greater in his generosity) was performing the bigger mitzvah. He uses this to explain the famous mishnah of “Eizehu oshir hasomeyach bichelko”. Simcha denotes the joy shared with another, and refers to tzedokoh. Check out the Mahara”l.