Is it obligatory to stand during the opening takbir for someone praying while seated on a chair due to a cast on their foot, and must previous prayers performed with the opening takbir while seated be repeated?

If the worshipper is able to stand, it is obligatory for him to do so. His inability to prostrate due to illness does not waive the obligation of standing or the rest of the attainable pillars. He should have performed the takbir and recitation while standing, unless he was unable to do so. If he is unable to stand for recitation, he must stand for the opening takbir, then sit and perform the rest of the pillars according to his ability. A prayer performed sitting for takbir or recitation out of ignorance does not need to be repeated unless he becomes aware during the time of its obligation to repeat it.