Is it permissible to receive money in cash from virtual money trading competitions, or to accept it as money in a trading account without using the margin system?

Trading currencies involves many Sharia-related caveats, and it is more problematic than other forms of selling. As for the competition mentioned, we have no knowledge of it. The basic principle is to prohibit stipulating compensation for a race in anything other than what is explicitly stated in the texts or what falls under their meaning. What you have mentioned is neither explicitly stated nor does it fall under the meaning of what is stated. Therefore, engaging in such a competition is forbidden.