The Ivory Coast international helped Rangers reach the Europa League final and win the Scottish Cup during his temporary spell with the Glasgow club for the second half of last season.
Amad scored three times during his spell at Rangers, including a goal just four minutes into his debut against Ross County.
He also scored early in his United career with a fantastic header against AC Milan in the Europa League round of 16 and found the net again in our home pre-season friendly with Rayo Vallecano last month.
The 20-year-old went to the Tokyo Olympics, representing Ivory Coast, helping his country to reach the quarter-finals of the tournament.