I run proxy through RPi3b+ that is installed in another country. I want to open a port on RPi that is being used by qbittorent on main machine which is 6881. Is there any way to do it?
Probably not... but it might depend on what you're actually attempting to do.
What is port 6881 in qBittorrent? Is that a proxy port that qBittorrent is using, or are you referring to something else?
And what is it you want to do with the open port on the RPi?
The reason I ask is that qBittorrent does not have any support for incoming bittorrent connections when using a proxy. In other words the qBittorrent Listening Port is ignored when you put it into Proxy mode. Using a proxy in qBittorrent means never having an open port for incoming bittorrent connections, you're always firewalled / not connectable in that configuration.
If that's what you're trying to do it won't work but maybe it could work in a future update eventually.. if you're trying to do something else you may want to expand in your post (?)
e.g.
https://github.com/qbittorrent/qBittorrent/issues/21134
https://github.com/arvidn/libtorrent/pull/7726
https://github.com/arvidn/libtorrent/pull/8003
PS - Some other communities you can try