d request no. 5 for the year 2004 which arrived via e-mail and which includes the following:
What is the ruling for buying the sacrificial animal by weight i.e. weighing it and calculating its price per kilogram? Please note that the price of meat per kilogram is predetermined. There is a prevalent opinion in our country on the non-obligatoriness of buying the sacrificial animal by weight and paying for the [rent of the scales]. May Allah reward you well!

There is no legal objection to buying the sacrificial animal by weight and paying its price per kilogram before slaughtering it as there is likewise no objection to paying for the cost of weighing it.
The answer to the question is known from the above.