Bayern Munich chairman Karl-Heinz Rummenigge has revealed that the club took advantage of uncertainty at Manchester United when agreeing to sign Renato Sanches with a "handshake."

Bayern confirmed the signing of Sanches, 19, from Benfica for an initial €35 million in May, before he went on to impress as Portugal won Euro 2016.

And Rummenigge told Sport Bild that the Bavarians "reacted quickly" after United had failed to follow up their initial interest in the midfielder.

"We knew very well that Renato had a firm offer from Manchester United," he said.

The Bayern deal started to take shape on the eve of the club's Champions League semifinal defeat in Madrid against Atletico Madrid in April.

"I heard that the United deal couldn't be completed, as they didn't know who their next manager would be," Rummenigge added. "We invited Benfica's president and Sanches' agent to Madrid and that night we shook hands on the transfer."

Sanches' agent Jorge Mendes is reported to have sent a text message to Rummenigge after his client scored for Portugal in their Euro 2016 quarterfinal success against Poland in the summer, writing: "You can be really happy doing the business really early."

Renato Sanches joined Bayern Munich from Benfica in the summer. TF-Images/Getty Images

Rummenigge said that Sanches was a Carlo Ancelotti recommendation before the Italian was installed as Pep Guardiola's successor as Bayern boss.

Ancelotti has since labelled the teenager "a phenomenon" and "one of the best players at Euro 2016."

Meanwhile, Rummenigge is confident that Xabi Alonso, Arjen Robben and Franck Ribery will all extend their stay with the club.

Alonso, 34, Robben, 32, and Ribery, 33, are only contracted with the German champions until the end of this current season.

"We are speaking to them. It's conceivable that we'll continue with all three of them," Rummenigge said. "Ancelotti is renowned for his ability to work with older players."

Rummenigge, a former Bayern forward with 217 goals in 422 games, was also optimistic about the club's prospects of retaining Robert Lewandowski's services.

"I'm convinced that Robert will stay with us for a long time," he said. "Besides, he's still under contract for three more years."