One of the conditions for Atamatu’ with Umrah to Hajj is that the performer is obligated to have a sacrifice after performing the Umrah of that year in Mecca, he does not go out of it. If he comes out of it, then he does not have to slaughter, and that is why The Truth, Glory be to Him, says: “then whoever performs ‘umrah [during the Hajj months] followed by Hajj [offers] what can be obtained with ease of sacrificial animals”. [Quran 2 : 196 ]
The obligation of sacrifice is required for the one who disengaged from Ihram of Umrah, then he stays in Mecca throughout the same year until he performs the Hajj. For indeed this is affirming that he’s Al Mutamatu’