Is it permissible to make Salat in a Masjid that is under mortgage with Riba is involved? Can you pay the principle of that mortgage to relieve that burden from your brothers?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

It is forbidden for Muslims to purchase a place for a mosque through any interest-based loan. If they have committed that sin, they should hasten to make sincere repentance. However, performing prayers in such a mosque is permitted and they will be considered as valid prayers.

In addition, supporting the people of this mosque to settle their installments is permissible.

Allah knows best.