Once again Barcelona are psending the summer battling with a wall which until now has been impassable: Pairs Saint-Germain, backed by money from Qatar since they were taken over in 2011.

Midfielder Marco Verratti is the player Barça want this summer. The club feel he is the player they need in midfield for Ernesto Valverde's side to re-find success in La Liga and the Champions League.

In principle, Barça's problem is that PSG are a buying club. Owned by Nasser Al-Khelaifi, their aim is to conquer European football, win the Champions League and become known as one of the continent's elite sides.

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The investment in signings at the French club has been huge so far. They have only seen the brakes put on, so far, by the desire of players such as Leo Messi, Neymar and Cristiano Ronaldo to stay at Barça and Madrid to play in a bigger, better league.

That doesn't mean that PSG haven't dominated the transfer market in recent seasons. And the most important thing from a Barcelona perspective is that they've imposed themselves on more than one signing.

A salvo, pero...

Barça have managed to keep Neymar and Messi away from PSG, but they've not been able to lure the French side's biggest players to Camp Nou, such as Thiago Silva or Marquinhos. They've not been able to lure their promising youngsters, either, such as Rabiot.

Now history is repeating itself with Verratti. The Italian, who has grown as a footballer in Paris, wants to come to Camp Nou to play alongside Messi and to win titles.

PSG's failings in the Champions League -- especially against Barça, such as the 6-1 this season -- have led to Verratti openly asking the club's hierarchy to let him go, according to ‘La Gazzetta dello Sport’ a few days ago.

The response from PSG, once again, was no. And that's before Barcelona were even able to begin talks or make their interest in the player known officially.

A situation similar to five others:

Thiago Silva: Joined the French club from Milan in 2012 for 42 million euros and became the captain and symbol of the new PSG. Barça tried to sign him for several summers by using the player's desire to come, but contract renewals kept him in Paris.

Marquinhos: Summer after summer it was the same story with the Brazilian. In this case, Marquinhos represented a bet for the future, the heir to Thiago Silva. But PSG 'broke' the market pating 31.4 million euros to Roma for him in 2013 when he was 19. He began as a substitute in Paris and was keen to listen to Barça's offers, but Nasser Al-Khelaifi, once again, said no.

David Luiz: Another Brazilian centre-back at PSG, more Barça interest. The Catalan club were looking for a defender but were unable to get over the wall in Paris. PSG spent 49.5 million euros to sign Luiz from Chelsea, before selling him back to Stamford Bridge for 35 million euros last summer.

Adrien Rabiot: The young French international (22) came through the club's academy and broke into the first team after a loan spell with Toulouse a few seasons ago. Barça saw the midfielder as an option for the future and someone who could complement Sergio Busquets, but were never able to get talks going with PSG.

Angel Di Maria: The Argentine left Real Madrid for Manchester United for 75 million euros but didn't adapt to the Premier League. He quickly became the object of various other clubs, among them Barça, who knew he worked well with Messi. PSG, though, closed a deal for 63 million euros, but Di Maria is still linked with a move to Camp Nou.

Marco Verratti: And now for the 2017 market. Verratti feels his time at PSG has reached an end and has told the club he wants to go. Barça have began talks and are ready to pay a big fee, up to 80 million euros... but the French club, once again, are refusing to negotiate. We will see if Marco is capable of moving things forward.