Well it's haraam for Muslims to fight each other and take each others properties.
But besides the scenario, it's permitted to have Muslim/Mumin slaves. Allah speaks about believing slaves in several ayahs of the Quran.
In Surah al-Baqara 2:221 =
And do not marry polytheistic women until they believe. And a believing slave woman is better than a polytheist, even though she might please you. And do not marry polytheistic men [to your women] until they believe. And a believing slave is better than a polytheist, even though he might please you. Those invite [you] to the Fire, but Allah invites to Paradise and to forgiveness, by His permission. And He makes clear His verses to the people that perhaps they may remember.
Surah an-Nisa 4:92 =
And never is it for a believer to kill a believer except by mistake. And whoever kills a believer by mistake - then the freeing of a believing slave and a compensation payment presented to the deceased's family [is required] unless they give [up their right as] charity...
So Allah speaks about believing slaves while they are enslaved. There's nothing wrong with having slaves in general as long as you treat them properly and give them their rights.