If you're into charity, donate from your commission and not from people's winnings.
If you're into righteousness, build a church or whatever.

You're a betting platform so I hope I don't have to explain to you how the betting world works. You can't call something matchfixing simply because you don't like it.

The counterpart in this bet should not be compensated in any case considering that he lost the bet. What kind of idea is it to begin with?

If you want to protect yourselves and your players, maybe don't offer the possibility of betting crazy amounts on amateur sports? That would decrease your commission though, that's probably why it didn't cross your mind.

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This looks like a simple case of you needing to return funds to the player and it would be good for your business that you speed up the process of doing it.
You risk losing your reputation with the arguments you've provided so far.

I do support arbitrators (plural, not singular), but as I said - I don't think they are needed at all.