Does praying while sitting due to a chronic back illness reduce its reward by half?

"If a Muslim servant falls ill or travels, the reward for what he used to do when healthy and resident is written for him. This indicates that abandoning standing in prayer due to illness or similar reasons does not diminish the worshipper's reward, because he is excused. So, whoever has an excuse, such as illness or otherwise, and prays sitting, will have a reward similar to that of one who prays standing."