In chapter 15, what does Holden do for the nuns? Is it true to Holden's personality?

Holden, having made a date with Sally Hayes (whom he thinks of as a phony), sees two nuns with cheap suitcases in a place where is having breakfast. One nun is carrying a straw basket of the type used to collect money for charity. Although the nun says it is not a charity basket, Holden nevertheless wants to donate money to them. He says,

They let me give them ten bucks as a contribution. They kept asking me if I was sure I could afford it and all. I told them I had quite a bit of money with me, but they didn’t seem to believe me. They took it, though, finally.

This is completely in character for Holden. He talks about being careless about money and often forgetting to collect his change after paying a bill, as well as losing money. He attributes this to having a wealthy father. It becomes clear, however, that his wealth, compared to other people's, makes him feel guilty. He says that money makes him feel "blue" or sad.
The nuns' cheap suitcases, as well as the fact they are only eating toast and coffee, when he has had a huge bacon and egg breakfast, makes him want to do something for them. We learn that one of his roommates also had cheap suitcases and kept them under the bed out of shame, so Holden started doing the same with his. He thinks that he would have been willing to trade his suitcases for the roommate's. He wishes too that he had given the nuns more than ten dollars. His money weighs on him, and he would like to use it to help people. Nevertheless, he appreciates his conversation with nuns, telling them he'd "enjoyed talking to them a lot, too. I meant it, too."

In chapter 15 of The Catcher in the Rye, Holden donates $10 to the nuns he talks to at the cafe and attempts to pay for their bill. However, they do not let him. Holden's action absolutely fits with his character, as he enjoys those who are not phony (particularly children). He sees the nuns as innocent, as evidenced by his comment about feeling uncomfortable talking to the nun who happens to be an English teacher about Romeo and Juliet because "that play gets pretty sexy in some parts."
In other parts of the novel, Holden shows charity and kindness toward those who need it. He buys his sister an album, sells his typewriter for $20 even though it cost $90, and helps a little girl at Central Park with her ice skates.
Ultimately, the nuns are merely symbolic of Holden's desire to protect the innocent. His action falls completely within his character.