Should I share my external grant with my supervisor? He reasons that he supported me earlier I secured a grant from my university for traveling to a conference. But it was not enough and so my supervisor supported me. There are 5 other students who got the university grant and he supported them as well. I had also applied for ACM grant and I secured it but I receive it only after I visit the conference.

Now my supervisor has ordered me to give the complete ACM grant to him because he supported me. I am not sure if it is ethical. I wish to use this money for another conference which I plan to attend. I am OK with giving the money to him or school, but since I got the grant it should be spent on my endeavors and not be used by anybody else. Am I wrong to think like this?

Edit : My application to ACM grant was independent. I wrote the application myself without support from anyone. In fact, the paper which I wrote was also written by me, with some changes from another supervisor. The idea was completely mine, as well - with no inputs from anybody else.

Edit : In the light of this event and other instances of being bulleyed into doing things which are unacceptable to me I have decided to quit working with my supervisor. Does this make any difference? If at all I have to return, I wish to return the money to university and not to this supervisor.
Take into account that ethically (and often legally) speaking you cannot be reimbursed more than once for a conference trip.

I am assuming that the ACM grant was a travel grant to cover the expenses for the conference, right?

The most reasonable option, given that you found external funding, is to return the money from the university grant + your professor's, using that same ACM grant.

It's not your professor's business to decide what you do with the remaining money from that grant, if there is any.

Your professor should actually be happy that you were able to get external funding so you could attend at 0 cost for the University and his team.

Saying that you have to pay back more to him than he "loaned you" because "he supported you" tells a lot about your professor already: he does not support or help students because he considers it's his duty or job, but in exchange for future rewards.

Unfortunately, this bullying, favor/grant-exchanging, mob-behavior is not new to many of us at all.