Praise be to Allah the Lord of the Worlds.
It isn`t permissible to buy sheep or other cattle based on the method given in the above question: a certain price for the kilo, because the amount of the meat before the animal is slaughtered is unknown, an this makes the buyer a victim of ignorance and uncertainty, which are amongst the nullifiers of sale contracts.
However, the buyer could make a promise to buy the animal`s meat against a certain price for the kilo, after it is slaughtered. The sale transaction takes place upon weighing the meat where the amount and the price become known, and this is permissible in Sharia.
In order for the sale contract to be valid, scholars stipulated that the commodity and the price are sighted. Al-Shirbini says: "It is valid to sell a heap of food whose amount is unknown to buyer and seller, against one Dirham for each Saa`. The validity of this transaction is derived from the fact that the food is sighted and the buyer`s ignorance of the total price doesn`t invalidate it, because the price is known in detail: one Dirham for each Saa`." {Moghni al-Mohtaaj, 2/355}.
As for the Udhiyah, Aqiqah and vowed animal, they must be made in the possession of the buyer before they are slaughtered, and it isn`t permissible that they get slaughtered while possessed by the butcher. They must be bought alive, then slaughtered as an Udhiyah or else. And Allah the Almighty knows best.