Is it permissible for the giver of charity to ask the recipient of charity to supplicate for him, and does that diminish the reward of the charity?

There is no harm in asking the recipient of charity to supplicate for one, and the Sunnah is for the recipient to supplicate for the one who gave charity, based on the Almighty's saying: {Take, [O, Muhammad], from their wealth a charity by which you purify them and cause them increase, and invoke [Allah's blessings] upon them. Indeed, your invocations are reassurance for them.} [At-Tawbah: 103]. Likewise, the rewards of your charity do not diminish if the recipient supplicates for you. It is more complete to supplicate for them with something similar to what they supplicated for you, so that your reward remains abundant.