Is it permissible to swear an oath to reclaim a disputed property when the original agreement was for temporary usufruct and not a formal lease, and is the oath considered legitimate if it prevents the perpetuation of injustice and continued usurpation?

It is permissible to swear an oath regarding what transpired between you, namely that the lease was temporary and not official, because an oath was legislated to repel injustice and recover rights. As for swearing an oath on something other than what occurred in order to recover a right, some scholars have permitted it through interpretation and dissimulation (tawriya), whereby the one swearing the oath intends a true meaning with his utterance, even if the apparent meaning of the word suggests a lie. However, resorting to lying is not appropriate when dissimulation is possible.