ESPN reported on Monday that City had agreed a €65 million ($71m) deal with Rennes to secure the services of the 21-year-old.
Doku has scored 12 goals in 91 games for Rennes since arriving from Anderlecht in 2020. He has been capped 14 times by Belgium, scoring twice, and was part of the squad for the 2022 World Cup.
With just over a week remaining of the summer transfer window, City's business is unlikely to end with Doku.
Defenders Aymeric Laporte joined Al Nassr earlier on Thursday, and João Cancelo is set to depart to Barcelona. The club are also seeking a replacement for injured midfielder Kevin De Bruyne, with the 32-year-old ruled out until the new year after undergoing surgery on a hamstring injury.
Sources have told ESPN that City have been given encouragement that Wolverhampton Wanderers will listen to a second offer for Matheus Nunes but will walk away from a deal if there is no compromise over their valuation of the Portugal international.
City saw a £47 million ($59m) offer turned down on Wednesday with information coming back that it fell way short of Wolves' valuation of the midfielder.
The Premier League champions have won the opening two games of their title defense, but will be without manager Pep Guardiola on the sidelines until after the September international break after he underwent back surgery in Barcelona on Tuesday.
Information from ESPN's Rob Dawson contributed to this story.