All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Slaughtering the mandatory sacrificial animal before the Day of An-Nahr fulfils the obligation according to some scholars like the Shaafi‘s and a reported opinion of Imaam Ahmad . This is if the pilgrim entered Makkah before the ten days of Thul-Hijjah and it was difficult for him to keep the sacrificial animal. Imaam Ahmad was also reported to have permitted such early slaughtering unconditioned. In support of the view held by the Shaafi‘ scholars, An-Nawawi quoted as evidence the verse (what means): {And when you are secure, then whoever performs 'Umrah [during the Hajj months] followed by Hajj [offers] what can be obtained with ease of sacrificial animals. And whoever cannot find [or afford such an animal] - then a fast of three days during Hajj and of seven when you have returned [home].}[Quran 2:196] As scholars agree that it is permissible to fast before slaughtering, then it is worthier to allow slaughtering before standing on ‘Arafaat.
However, the Maaliki, Hanafi, and Hanbali scholars maintain that early slaughtering does not fulfill the obligation. Hence, the pilgrim should slaughter on the day of An-Nahr to conform to the Sunnah and stay away from the difference of opinions among scholars.
Allaah Knows best.