At first, Allah’s Command was that they had to slaughter a cow, without specifying its description, because the purpose was to show them His Miracles. By striking the dead man with a part of it, Allah would revive him, so he could guide them to his killer, and the problem would be over.
However, whenever they asked Moses about its description, Allah the Almighty made the matter a little difficult for them. They did not stop asking for more descriptions, in order to avoid complying with Allah’s Command due to their arrogance towards Moses. Had they obeyed Allah’s Command from the very beginning, the matter would have ended. The cow was normal and available.
كان الأمر من الله تعالى في البداية أن يذبحوا بقرة و لم يحدد الله أوصافها فكانت الغاية أن يريهم إعجازه بأن يضربوا الميت بجزء منها فيحيي الميت ليدلهم على قاتله، و تنتهي المشكلة.
لكنهم كلما سألوا موسى عن وصفها كان الله تعالى يصعب عليهم الأمر قليلا. فلم يتوقفوا كانوا لا يريدون الامتثال لأمر الله تعنتا منهم على موسى. لأنهم لو طبقوا أمر الله أول مرة لكان الموضوع انتهى. فكانت البقرة عادية و متوفرة.