Barcelona still have work to do to get a deal for Ajax centre-back Matthijs de Ligt over the line despite the optimism they had one month ago, sources have told ESPN FC.

Barca had grown confident of landing De Ligt this summer after striking a loose agreement with him in March, but his agent Mino Raiola is now trying to get the Catalan club back to the negotiating table.

The interest in the Netherlands international -- Bayern Munich, Juventus and Manchester United have all been linked -- and Ajax's run in the Champions League have led Raiola to suggest that the previous pact is no longer suitable.

Barca remain interested in doing a deal for De Ligt, although a club source has told ESPN FC they "will not go crazy" when it comes to matching any new demands Raiola may put forward. The Catalan club have a spiralling wage bill and do not want to push their salary limit even closer to the edge.

Contacted for comment by ESPN FC, Raiola said he preferred not to talk about De Ligt's future at the moment.

Ajax and Barca have already agreed on a potential fee which could rise to €80 million with add ons, as long as there is also an agreement with De Ligt -- whose intention is to move to the Spanish champions ahead of other interested parties.

Since completing a deal for Ajax midfielder Frenkie de Jong in January, the Blaugrana have focused all their attention on De Ligt. However, a high ranking executive at the club has told ESPN FC there is still plenty of work to be done and that they are not taking anything for granted following the unexpected departure of Neymar in 2017 and their failure to sign Antoine Griezmann last summer.

Talks remain ongoing with Raiola, who may look to take advantage of Ajax's impressive season. Like Barca, they could end the campaign with a Treble. They have already won the Dutch Cup, are top of the Eredivisie and are still in the Champions League.

Barca's original offer to De Ligt included a five-year contract which would see him earn more in each year of the deal. It also included a series of bonus payments based on objectives such as games played and titles won, which were key to the agreement.

The club are now wary that any new terms would see him earn more than his Ajax teammate De Jong will at Camp Nou.

Barca and Raiola have parked negotiations until next week for now, with the club focusing on Tuesday's match against Liverpool -- and De Ligt and Ajax preparing for Tottenham's visit to Amsterdam on Wednesday.

"We wanted all the focus this week to be on the Champions League semifinals, which Barcelona and Ajax are both involved in," the club source said.

It is hoped everything will be resolved before the Champions League final on June 1 -- which could pitch Barca and Ajax against each other.

Ajax, meanwhile, are already preparing for the future without De Ligt. The cash they will receive will allow them to sign a replacement and they have turned their attention to Edson Alvarez, who plays for Mexican side America.