Sanchez signed with Inter on loan at the start of the 2019-20 season following a disappointing spell at United where he scored only five goals in 45 matches.
Although the Chilean scored only four league goals this season, he seemed to have rediscovered his touch in Italy where he topped the club's charts with eight assists as Antonio Conte's Inter finished runners-up behind Juventus.
"It will be official tomorrow [Thursday] in the morning," Marotta told Sky Sports.
"The player is completely ours and we have a three-year contract with him. We think he can be a full member of our squad."
The transfer will also allow Sanchez to play in the rest of Inter's Europa League campaign after they advanced to the quarter-finals on Wednesday with a 2-0 win over Spanish side Getafe in Gelsenkirchen, Germany.
Following Manchester United's win over LASK in the Europa League, manager Ole Gunnar Solskjaer also confirmed a deal was in place.
"We'll announce it tomorrow [Thursday]," Solskjaer said. "He's agreed and we've agreed. He enjoyed his time there and they've liked him. He's a top player who we want to see playing his best football. We didn't see the best of him for whatever reason. He's a top professional, now we just wish him all the best."