Is the commission I received for finding accommodation for the tenant, and his subsequent annulment of the contract with the landlord, forbidden to me?

Brokerage is a permissible activity, and the broker is entitled to their full fee upon completion of their work. Whoever carries out the agreed-upon work deserves the stipulated payment, just like a debt. In this case, the questioner has completed their work, and thus is entitled to their fee. The other party has no right to demand its return, even if they leave or terminate their rental agreement with the property owner.