What is the ruling on the posture of the foot in prostration when there is pain or difficulty that prevents one of the feet from being upright during the sitting between the two prostrations?

The manner of sitting in prayer is among the Sunnah, not obligatory. If a worshiper is unable to sit in the Iftirash position due to difficulty or other reasons, there is no blame upon him, and he may sit in a way that suits his condition. His prayer is valid.