What is the ruling on purchasing the Udhiyah (sacrificial animal) in mid-Dhul-Qa'dah without specifying the price, then agreeing upon the price and paying at the market rate on the day of Eid after the slaughter?

The agreement to reserve a sacrificial animal is a promise to buy, not a contract, and it is permissible not to fulfill it for a benefit. Buying the sacrificial animal on the Day of Arafah and deferring payment is valid, but the majority of scholars stipulate knowing the price at the time of sale, contrary to Sheikh al-Islam Ibn Taymiyyah, who holds the view that the sale is valid without specifying the price, and it is calculated at the market price. So, if the price is not specified, the sale is valid according to the madhhab (definition) of Sheikh al-Islam Ibn Taymiyyah. As for the view of the majority, the responsible person sins if he engages in invalid contracts, therefore the price must be determined on the day of purchase in the future.