Is a witness considered sinful and must he offer expiation if he swears a false oath to establish a legitimate right?

Your duty was to establish the company's rights without lying or false oaths. You could have used allusions and dissimulation (tawriyah). As for outright lying and swearing to it, it is permissible only if it is the sole means to obtain a right and dissimulation is not possible, and this must be within the narrowest limits. Any praiseworthy objective that can be achieved without lying makes lying impermissible in that instance. However, if it cannot be achieved except through lying, then lying is permissible. Thus, lying becomes permissible to achieve something permissible, and obligatory to achieve something obligatory. If attaining the right was possible without lying, then you have sinned and must repent. If it was the only determined way, then there is no blame.