The I Gotta Feeling hitmaker has worked with artists including Michael Jackson, Britney Spears, Sir Mick Jagger and Jennifer Lopez among others, and he admits the movie star is the next big game he wants to record with.
He says, "I would like to work with Robert Downey, Jr... That guy sings."
The actor has sung on several soundtracks and he released an acclaimed album in 2004, titled The Futurist.
The record shop listed comeback singer Amelia's single on their website today.
The chain's site said the untitled single would be released on Monday December 12 through record label Syco - owned by X Factor mogul Simon Cowell.
Retailing at ?3.99, the website said the CD would be shipped in "three to six working days".
But the company's website didn't offer fans the opportunity to pre-order potential winner's singles from Amelia's fellow finalists Little Mix and Marcus.
One fan, who spotted the single for sale, said: "I was pretty shocked - I was just browsing the upcoming releases and spotted it.
"I assumed it was something that would allow you to buy the single of whoever went on to win.
"But the only option was to buy Amelia Lily's song - there was no option for Marcus or Little Mix.
"I had to wonder whether the result of the show was known somehow by the store. I was shocked."
On Twitter, stunned fans claimed the result of the TV talent show had been fixed.
User racface wrote: "So....why is Amelia Lily's XFACTOR WINNERS single ready for purchasing on HMV and Marcus and Little Mix are not.Big Fix!"
JessicaLucyA wrote: "Hmmm.... people who still dont think x factor is a fix, does this convince you??"
And acciogeorgia said: "Wait what, Amelia Lily's winner's single is on the hmv website? lol the Fix Factor strikes again."
A spokeman for HMV today said: "It was an error. We are putting all three singles up so people have a chance to pre-order them all. But only the actual winner's song will be processed."
JLS are reportedly being lined up to record the official London 2012 Olympics song.
Oritsé Williams revealed that he and fellow bandmates Aston Merrygold, Marvin Humes and JB Gill have written a track for the sporting event with producer Ali Tennant.
"I'm not supposed to say anything right now but we have written a song and are talking to people about it being the Olympic theme,"
"It is perfect, we were asked to do it. The song is uplifting, it's kind of along the lines of R Kelly's 'I Believe I Can Fly'. We wrote it with Ali, who is brilliant."
JLS will also feature on this year's X Factor charity single 'Wishing on a Star' alongside the current contestants and One Direction.
The band are due to perform on The X Factor results show tonight, singing new single 'Take A Chance On Me'.
Speaking of bandmate Merrygold, Williams added: "He can't speak at the moment, he has had to rest his voice. There has been a lot of colds going around so we haven't been out on the pull."
