Is one rewarded who cannot stand for long due to allergies, but forces himself and endures the pain?

It is permissible for someone who finds it difficult to stand during prayer to pray while sitting. Their reward will not be diminished compared to someone who prays standing if they have a valid excuse. The difficulty that permits sitting is that which causes harm or removes khushu' (devotional humility). Fatwas no. 159395 and 123623 have clarified the limit of such difficulty.