Is it permissible to hire false witnesses to prove ownership of usurped real estate, if the real witnesses refrained from testifying for fear of the aggressors?

It is legislated to strive to reclaim a fixed right, but this must be done through legitimate means, not by presenting false witnesses. Presenting two witnesses who have no knowledge of what they are testifying to is a grave sin in the sight of Allah, and false testimony is one of the major sins. The Prophet, peace and blessings be upon him, forbade treachery, even if it were to reclaim a right, saying: "Return the trust to the one who entrusted it to you, and do not betray the one who betrayed you."