Manual of Islamic Law – Sunan & Nawafil Namaaz
Law: If one performs his Nafil whilst sitting, he should sit like one sits in Tashahud,
But when in the condition of Qira’at the hands must be tied under the naval just as it is tied when in Qiyaam. [Durr-e-Mukhtar, Raddul Muhtar vol.1 pg.654]
Law: If one took a vow that he will perform Namaaz at certain place and he performed it at a place which is not as excellent, the Namaaz is still discharged.
For example, If one intended to perform it at Masjid-e-Haraam but rather one performed it at Masjid-e-Quds or the Masjid of one’s home (local), it is valid (discharged).
If a female took a vow that the next day she will perform a (particular) Namaaz or keep fast and on that day she began menstruating, then she should perform Qaza of these. If she made a vow that she will perform 2 Raka’ats in the condition of Haidh, then there is nothing (i.e. no Qaza).
Law: If one took a vow that one will read 2 Raka’ats today, and he did not read it today, then there is no Qaza for it, but one will have to give kaffarah (compensation). [Alamgiri vol.1 pg.115]