Is it permissible for a sick person to pray while sitting on a chair?
All perfect praise be to Allah, The Lord of the Worlds It is permissible for a sick person, who is unable to stand up, to offer prayer while sitting on a chair. If he/she was unable to prostrate while on the chair, the he/she should pray while sitting on the ground, but if he/she was neither able to bow (Roku),nor to prostrate, then he can do them while sitting on the chair. And Allah Knows Best.
Is it permissible to pay the expiation of an oath to a charitable party as it feeds the poor and needy such as Tkiyet Um Ali?
There is no harm on authorizing someone to handle the expiation of another to the poor and needy, since Shafiee's jurists stated that authorizing someone for the purpose of distributing Zakah, expiation and vow is permissible as stated in [Mughni Al-Muhtajj,vol.3/pp.237]. At last, the intention of the person who intends to authorize someone else suffices in this regard. And Allah Knows Best.
Is it permissible for a woman who is in her confinement to fast upon seeing blood signs that are neither black, nor red ?
Brownish, reddish, and yellowish discharges are all considered confinement until there is no colored discharge which marks the end of confinement. Moreover, the aforesaid woman is prohibited from fasting until she attains ritual purity. And Allah Knows Best.
In the last verse of Surah At-Tin, Allah Says (What means): "Is Allah not the most just of judges?" [At-Tin/8]. Is it permissible for us, while performing congregational prayer, to respond after the Imam recites it by saying: "Indeed, and I am among those who bear witness to that." ?
It is recommended for the worshipper to say 'Indeed' (Bala) silently after reciting or hearing the verse (/What means): "Is Allah not the most just of judges?" [At-Tin/8]. This is part of reflecting on the Quran during its recitation or listening, as Allah Says (What means): "Then do they not reflect upon the Quran, or are there locks upon [their] hearts?" (Muhammad: 24). Doing so helps in achieving humility and focus in prayer. And Allah Knows Best