What is the semi-circular structure attached to the Holy Kabah?
The semi-circular structure attached to the Holy Kabah is Hijr Al-Kabah (is a low wall originally part of the Kabah) and passing behind it is an obligation while making Tawaf because it is a part of the Holy Kabah. And Allah Knows Best.
What is the validity of the formula for sending blessings upon our master Muḥammad ﷺ: *"Allāhumma ṣalli wa sallim wa bārik ‘alā sayyidinā Muḥammad wa ‘alā ālihi, ‘adada kamāli-Llāhi wa kamā yalīqu bikamālihi"* ("O Allah, send blessings, peace, and grace upon our master Muḥammad and his family, by the number of Allah's perfection, and as befits His perfection")?
All praise is due to Allah, and peace and blessings be upon our master, the Messenger of Allah ﷺ.
It is permissible to send blessings upon our master, the Prophet ﷺ, using the formula mentioned in the question, since sending blessings upon the Prophet ﷺ is an act of devotion legislated in its basic essence, without a specific prescribed wording required outside of the formal prayer itself. It is therefore valid to perform it outside of prayer using any formula. This particular formula involves multiplying the blessings sent upon the Chosen Prophet ﷺ to the number of the perfections of Allah, the Exalted — which are limitless — so that, by Allah's permission, its reward and benefit never cease.
The meaning of "the number of Allah's perfection" is that it is without limit, making the blessings sent boundless, corresponding to His perfection, Glorified is He, which cannot be enumerated or counted. This formula is analogous to the words of our Greatest Prophet ﷺ, as related in the authentic ḥadīth narrated by Imam Muslim: the Prophet ﷺ said to Sayyidah Juwayriyyah (raḍiyallāhu ‘anhā): "Indeed, I said four phrases after you [left], three times, which, if weighed against everything you have said since this morning, would outweigh them: Subḥān Allāhi wa biḥamdihi, ‘adada khalqihi, wa riḍā nafsihi, wa zinata ‘arshihi, wa midāda kalimātihi" (Glory be to Allah, and praise be to Him, by the number of His creation, by His pleasure, by the weight of His Throne, and by the extent of His words). And Allah, the Exalted, knows best.
What is the ruling of Islamic Law on selling gold or silver in installments or for a differed price?
Praise be to Allah, and peace and blessings be upon our Master, the Messenger of Allah.
It is prohibited (haram) to sell gold or silver in installments or for a deferred price; rather, immediate hand-to-hand exchange (Taqabud) is mandatory, otherwise, it is considered usury (Riba). The Messenger of Allah ﷺ said: 'Gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, and salt for salt—like for like, hand to hand. Whoever increases or asks for an increase has engaged in Riba; the receiver and the giver are the same in this regard.' (Narrated by Muslim). Furthermore, when exchanging new gold for used gold, they must be of equal weight, or it falls into Riba.
The permissible solution (al-makhraj) is for the merchant to purchase the used gold for cash first, and then sell the new gold for cash in a separate transaction. However, the price must be paid during the sitting of the contract (Majlis al-Aqd) for both deals. Alternatively, the jeweler may take the used gold with the intent of remodeling or repairing it, and then charge a fee for the craftsmanship or repair work. And Allah the Exalted knows best."
A person prays in a certain spot within the rows of the mosque, and gets angry if someone else prayed at his spot. What is the ruling on this behavior?
All Perfect Praise be to Allah The Lord of The Worlds and may His Peace and blessings be upon our Prophet Muhammad and upon all of his family and companions Prophet Mohammad (PBUH) make it prohibited for a Muslim from having/taking a certain spot in the mosque when performing prayers if they arrived and find it occupied, furthermore; it is the religious duty on others (praying people) to offer a piece of advice to alert the ones who don't know, that whoever reaches a spot first at the mosque having a priority/privilege upon others in that spot. And Allah Knows Best.