Manchester United will pay Nani’s wages – around £4.8 million – for the duration of his season-long loan to Sporting Lisbon.
The 27-year-old returned to Portugal as part of the deal which sees Marcos Rojo move to Old Trafford.
Sporting wanted the loan agreed before they would sanction Rojo’s £16m move.
Meanwhile, United have rejected reports that they were interested in signing Germany’s attacking midfielders Thomas Muller and Marco Reus this summer.
“It is not true,” said a United spokesman.
Nani joined United in 2007, since when he has won four Premier League titles and the Champions League.
He played in all three Portugal matches at this summer’s World Cup.
He was afforded a hero’s welcome on his return home on Tuesday.
Sporting play FC Arouca on 23 August and major rivals Benfica on 31 August.