What was Miguel’s reason for taking money?

Miguel took the money in order to reunite Esperanza with her grandmother. He used it to help Abeulita escape from Tío Luis and Tío Marco.

Miguel took the money, because he knew Esperanza & Ramona/Mama needed Abuelita, and Esperanza wanted proof things would get better, and so Miguel took the risk of going back and getting Abuelita just for them.