There is nothing wrong with selling live animals by weight
Question

There are people who sell animals such as cattle, sheep and the like, whilst they are still alive, by the kilogram for a fixed price. Please note that the purchaser wants to buy them alive so as to keep them with him or to slaughter them and sell them to people. For example: we go to the animal seller and choose what we want to buy, then he brings them to the scale and weighs them alive and sells them by the kilogram, for example 10 riyals per kilo. What is the ruling on that transaction?.
Praise be to Allah.We do not know of anything wrong with selling animals which it is permitted to sell, such as camels, cattle, sheep and the like by weight, whether they are alive or have been slaughtered, because of the general meaning of the words of Allaah (interpretation of meaning): “Allaah has permitted trading and forbidden Riba” [al-Baqarah 2:275]. And because when the Prophet (blessings and peace of Allaah be upon him) was asked: What kind of earning is best? He said: “For a man to work with his hands, and every legitimate transaction.” And because there is no ambiguity or deceit involved in that. And Allaah is the source of strength. End quote. 
Majmoo Fataawa al-Shaykh Ibn Baaz,(19/38)