What the giving does to the one who gives
Charity in the Qur'an carries a promise of return, and the return is described in farming terms: a single grain that comes up as seven ears with a hundred grains in each (2:261). The comparison is doing real work. Seed that a farmer buries has not left his possession, and the giver is asked to hold wealth the same way.
Alongside the promise sits a condition that gets far less attention. Wealth given and then held over the recipient's head is described as cancelled, the way a downpour strips the dust off a smooth stone and leaves it bare. A kind word ranks above a gift that wounds: "A kind word and forgiveness are better than charity followed by injury." (2:263). The dignity of the person receiving is part of what is being weighed.
The threshold is set at cost. "You will not attain righteousness until you spend from what you love" (3:92) locates the test in what the giving costs the one who gives. An amount that was wanted counts differently from an amount that was spare, and the same figure can be either depending on whose hand it leaves.
Concealment is encouraged without being demanded. Giving openly is called good, and giving quietly to the poor "is better for you, and it will wipe away some of your wrongs." (2:271). The reason is left unstated, though the surrounding verses about reputation and display make it hard to miss: a gift nobody watched cannot have been given for the watching.
The Qur'an also gives the poor a claim on wealth that is already theirs before anyone hands it over. Their portion is described as sitting inside the owner's holdings from the start, "a rightful share for the one who asks and the one deprived." (51:19). That wording settles the question of who is doing whom a service.
What the giving does to the giver is named directly — a charity "by which you cleanse and purify them" (9:103). The same reasoning reaches past money, so that a debt owed to you and written off counts as charity given (2:280). Receiving well and giving well are the two halves of one habit; see also verses on gratitude and verses on money and provision.