It is not valid to do the farewell tawaaf until after completing Hajj
Question

Is it permissible for the pilgrim to do tawaaf on the morning of the twelfth (of Dhu’l-Hijjah), then go back to Mina to stone the jamaraat and then leave for his homeland?.
Praise be to Allah.The farewell tawaaf is the last of the actions of Hajj, so it is not permissible to do it before any of the other rituals, because of the hadeeth of Ibn ‘Abbas (may Allah be pleased with him): “The people were commanded that the last thing they do should do is to circumambulate the House” 
And Allah is the Source of strength. May Allah send blessings and peace upon our Prophet Muhammad and his family and companions. End quote. 
Standing Committee for Academic Research and Issuing Fatwas