Q: Is it permissible to determine the price of the sacrificed animal after it has been slaughtered? That is buying the sacrifice by weighing it?
A:No it is not permissible. If one did this then the value of the animal would be binding on him because the sale is corrupt and whenever a sale is corrupt the value becomes binding when cancelling the sale is impossible. Also, the purpose is purchasing the animal for Qurbani. The method you mentioned implies that the purpose is the meat.