What does Eddie offer Dominguez? Why?

Eddie offers and gives Dominguez forty dollars to buy something for his wife on their trip to Mexico.

He watched Dominguez return to the sink. He thought for a moment. Then he took a small wad of bills from his pocket and removed the only twenties he had, two of them. He held them out.

"Get your wife something nice," Eddie said.

Dominguez regarded the money, broke into a huge smile, and said, "C'mon, man. You sure?"

Eddie pushed the money into Dominguez's palm.