Having already shelled out a lot for the Holland midfielder, their hands are tied due to Financial Fair Play rules.
Barcelona have been leading the chase for the in-demand centre-back for weeks.
A jewel from Ajax's endless conveyor belt of dynamic young talent, he is expected to follow in the footsteps of teammate Frenkie De Jong, who has joined Barcelona, and move to a big club in the summer.
Barca had wanted to conclude a deal this week, ahead of De Ligt joining up with the Holland squad for the UEFA Nations League - but that will no longer happen.
De Ligt celebrates scoring in the Champions League semi final.
Castles claims the winger is keen to join United in the summer window, saying: "Daniel James, the 21-year-old Wales worldwide at Swansea City, has been contacted by Manchester United, is very keen on that move understandably as he was in January on the verge of joining Leeds United, so the possibility of moving to Manchester United is obviously very attractive to him".
Oil jumps as Opec signals continued production cuts
Regardless of what OPEC+ decides next month, output in July won't exceed the kingdom's limit in the deal of 10.3 MMbpd, he said.
They deny that Liverpool have entered negotiations over De Ligt and "are not trying to secure the services" of the teenager, as Jurgen Klopp "is happy" with his current centre-half options. "So I think it would serve Liverpool well to go and get somebody beside Van Dijk who is tried and trusted".
Barca have worked on the deal for months.
Stam commented: "It's important that he'll get playing time".
Things can change quickly.
And although they were second best to Manchester City in the end, Liverpool are proud of the way they gave the champions a real run for their money by bridging the gap at the top.
De Ligt said: 'Will I play with him there?