If he took money to do Hajj on behalf of someone, and it was too little or too much, what is the ruling?

Praise be to Allah.

The Muslims are bound by their conditions, so if the giver and taker agreed that the taker would return any extra money and that the giver would make up any shortfall, then each of them must fulfil his obligations. If there was no such condition then he may take any extra money and he should make up any shortfall himself. But he will have the reward in sha Allah if he takes the money with a sound intention and fulfils the obligation.  

And Allaah is the Source of strength.