image sourceGetty Images

Manchester United have spent £12.85m on signing Cristiano Ronaldo, but they'll make that back in shirt sales as people rush to buy his kit, right?

That might be what your mates in the pub are saying, but it's not true.

There's a good chance the club will see a commercial benefit from the signing – on top of what Ronaldo does on the pitch of course – but shirt sales are not really part of that.

The top dogs at Adidas, though, will be rubbing their hands with glee.

  • 'I'm back where I belong' – Ronaldo
  • How Man Utd pulled off shock Ronaldo transfer

"The fee's already been agreed between the manufacturer and the club," says Dr Dan Plumley, who is a senior lecturer in sport finance at Sheffield Hallam University.

"So the shirt sales go to the manufacturer – or the majority of them [do]."

image sourceGetty Imagesimage captionJuventus fans left tributes to Ronaldo outside their stadium in Turin after news of him leaving the club

Manchester United signed a 10-year deal with Adidas worth £750m in 2014 – the biggest kit deal ever seen in world football.

The exact details of the deal are kept private so we don't know if United get a cut of shirt sales, but Dan says "it's very unlikely that that would ever be above 10%".

He says the club will get £5 per shirts at the absolute maximum – when the shirts on sale at the moment can cost fans up to £100.

Why are kit deals worth so much then?

Adidas may make back its £750m in shirt sales alone.

But Dan says there are other reasons they pay so much to make United's kits.

image sourceGetty Imagesimage captionSigning in style: Ronaldo and Kleberson show off their United kit in 2003

"There is a wider picture here of all the commercial aspects as well," he tells Radio 1 Newsbeat.

"Big brands like to be associated with other big brands," he says – and Manchester United and Adidas are two of the biggest brands in the world.

It's a similar story for Barcelona and Nike or Manchester City and Puma – they're mutually beneficial to each other.

Will Ronaldo get to wear number seven?

There have been rumours swirling around that Ronaldo will get his famous number seven shirt back.

He wore that at Manchester United, Real Madrid (except for his first season) and Juventus – and has built up his CR7 brand around it.

"He'll be wanting to protect that as much as possible, and the number on the shirt at Manchester United is another way to do that," Dan says.

There's a problem though.

image sourceGetty Imagesimage captionEdinson Cavani came on against Wolves at the weekend, wearing the coveted number seven shirt

Edinson Cavani already occupies the number seven shirt, so the club would have to make a formal request to the Premier League to swap numbers.

As it stands, there have been no pictures of Ronaldo posing with a Manchester United shirt and the Premier League website hasn't listed him with a number yet.

If Ronaldo gets the number that he made his name under, at the club he made his name with, there will no doubt be a surge in people wanting to buy his new kit.

United will hope his massive social media presence will help to build the hype even more.

  • Ronaldo tops Instagram rich list

"They'll be looking at adverts that target fans towards their own merchandise shop or come and visit Old Trafford, do the stadium tour and spend money in the club shop," says Dan.

"It's a mainstream global audience, and that can have some real impact."

But any money the club makes off that will still likely be very small in the context of their overall revenue, with which last year was £509m.

It still stands that the old "shirts pay for transfers" argument is just a myth.

Follow Newsbeat on Instagram, Facebook, Twitter and YouTube.

Listen to Newsbeat live at 12:45 and 17:45 weekdays – or listen back here.