Is complete inability to stand a prerequisite for the dispensation to sit during prayer, or is it permissible if standing causes harm? And what are the limits of the inability that permits sitting without sin?

Standing in the obligatory prayer is a pillar that may not be abandoned except in cases of inability, severe hardship, or fear of increased illness. The basis for this is the Prophet's (peace be upon him) saying to Imran ibn Husayn: "Pray standing; if you are unable, then sitting; and if you are unable, then on your side." The Ummah has unanimously agreed that whoever is unable to stand in the obligatory prayer may pray sitting, and there is no need for him to repeat it, nor will his reward be diminished. The hardship that permits abandoning standing is that which removes khushu' (devotional humility). The worshipper may sit cross-legged, or iftirash (resting on the left foot with the right foot upright), or ihtiba' (sitting on the buttocks with the knees raised and the arms or garments wrapped around the legs); the cross-legged position is preferable. As for supererogatory prayers, it is permissible to pray them sitting without an excuse, but the reward for one who prays sitting in such a case is half the reward of one who prays standing, unless his sitting is due to an excuse, in which case his reward is not diminished. As for the Prophet's (peace be upon him) praying sitting in supererogatory prayers despite his ability to stand, this is one of his unique privileges, as an honor to him.