All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
There is nothing wrong with buying the property on a fifty-fifty basis between you and someone else, whether for renting it or for another purpose. However, if you know that your partner in the other half will buy his share in the property via a prohibited transaction such as Riba-based mortgage; and that if you did not join him in that partnership he would not have conducted that prohibited transaction, then it is not permissible for you to be his partner for this involves tempting others and helping them in committing sin while Allaah The Almighty says (what means): {But do not cooperate in sin and aggression.}[Quran 5:2]
For more benefit, kindly refer to Fataawa 10549 and 81594.
Allaah Knows best.