About the ending
The ending is unacceptable to audience not just because Amal did not get the money, but the murderer was getting away with his crime. Now we can be pretty sure that Amal judging from his character most likely would decline the money even if he knew what was going on. In a way, the old man was crazy because he should have known (actually he knew) Amal cannot handle that kind of money. So Let us forget that part.
How about the part that justice was not served? I think that the director wants to show the unjust nature of Indian society at large. That is the environment lower class people like Amal lives in. There are many bad people on the top. As to the good people on top are incompetent to ameliorate the situation because they are alienated from the lower class (for example, the female lawyer did not expect Amal like many of her fellow nationals are illiterate). Therefore the movie is at the same time an indictment of the injustice of Indian society.
karlweb from Taiwan, Ming Dynasty