The midfelder is in his second season at Villa Park after arriving from Everton.
Onana's sister and agent Melissa spoke with Belgian transfer expert Sacha Tavolieri about the original move.
Melissa revealed: "Aston Villa's sporting project was crystal clear. It was a project where Amadou could continue to develop. We spoke with the sporting director, and the salary was also attractive, let's not kid ourselves.
"The Euros were coming up, Amadou was performing well, and we finalised the deal. Then, Manchester United entered the picture. But I had already given my word to Aston Villa.
"We knew very well that Manchester would be able to offer better financial terms, but we had already given our word to Aston Villa, and the project was a good fit.
"The style of football played, the club's ambition: Amadou felt right at home there."