Generosity and gratitude are two characteristics that go hand in hand. In the materialist world today, where people are running after having the most material or worldly possessions, having someone who could offer something to others to help them out is a big act of generosity. In return, if you do not have anything to offer, making dua for one who has offered you some of his wealth is an act of gratitude.
Islam emphasizes sharing and looking out for others. When someone shares their wealth with you, you should return the favor, but when you cannot, acknowledge it with gratitude and show generosity towards the person who is sharing.
Abu Hurairah narrated that Prophet Muhammad (ﷺ) said that those who do not thank the people have not thanked Allah (سُبْحَانَهُ وَتَعَالَى) [Sunan Abī Dāwūd 481]. This hadith clearly states that one must not feel entitled but rather be full of gratefulness for the way the people around have helped them in any way.
When making dua for one who has offered you some of his wealth, be expressive in your thanks and show gratitude to both Allah (سُبْحَانَهُ وَتَعَالَى) and the person who has been made a way in which Allah (سُبْحَانَهُ وَتَعَالَى) is providing you. Pray for their abundance with the genuine intention that Allah rewards them for their generosity and Allah puts you in a position where you are also able to share to help others. Offer the supplication with humbleness and the intention of reciprocating kindness.
The benefits of making dua for someone who has offered their wealth help both people. It opens a giver's heart to give more and share the good they are blessed with. For the receiver, it allows them to acknowledge the way that Allah (سُبْحَانَهُ وَتَعَالَى) is showing them generosity. It encourages the spirit of being helpful to others and being kind. It fosters bonds of generosity and kindness in the community and instills a feeling of gratitude in people for what they are receiving.