It is not permissible for the pilgrim who is doing tamattu‘ to do the sa‘i of Hajj before standing in ‘Arafah and halting in Muzdalifah
Question

A man who is doing Hajj tamattu‘  [i.e., ‘umrah followed by Hajj, exiting ihram in between] did ‘umrah, then he went to Jeddah, and on the eighth day [of Dhu’l-Hijjah] he came to Makkah and did tawaf and sa‘i with the intention of being exempt from having to do sa‘i on the day of Eid. Is he exempt from that, or not?
Praise be to Allah. 
It is well-known in the case of one who is doing tamattu‘, that he enters ihram for ‘umrah, does tawaf and sa‘i, then he cuts his hair and exits ihram. Then on the day of at-Tarwiyah [8
th Dhu’l-Hijjah], he enters ihram for Hajj and goes out to Mina; it will not benefit him if he does sa‘i before that, because it is not permissible to do the sa‘i of Hajj before standing in ‘Arafah and halting in Muzdalifah. However, if he is doing qiraan [Hajj and ‘umrah together, without exiting ihram in between], or ifraad [Hajj on its own], and he does sa‘i after tawaf al-qudoom [tawaf of arrival], then that is acceptable. Therefore, this man who did sa‘i ahead of time must avoid relations with his wife, if he has a wife, and he must travel to Makkah and do sa‘i, because he did it at the wrong time. It is best for him to enter ihraam for ‘umrah when he reaches the miqaat, and do tawaf and sa‘i, and cut his hair, then do the sa‘i of Hajj. "(Majmoo‘ Fataawa Ibn ‘Uthaymeen 22/87, 88)