Is the pilgrim who performed Umrah after his Hajj considered a Tamattu pilgrim, and is he required to offer a sacrifice?

The validity of Hajj al-Tamattu' is by performing Umrah before Hajj during the months of Hajj, and then entering into ihram for Hajj. So, whoever performs Hajj al-Ifrad and then performs Umrah after Hajj is not considered a Mutamatti' but rather a Mufrid. The Mutamatti' is a non-Makkah resident (afaqi) who enters into ihram for Umrah during the months of Hajj, performs its rituals, and then enters into ihram for Hajj and performs Hajj in the same year.