have been shocked by Newcastle's £50million valuation of midfielder Sean Longstaff.
United chief Ed Woodward was quoted the astronomical fee in negotiations with Newcastle's managing director Lee Charnley at the weekend.
It's understood that Newcastle have based their asking price on United's £50m purchase of Aaron Wan-Bissaka from Crystal Palace
, even though Longstaff only made his Premier League
debut in December and has a total of 16 appearances for the club.
United are reluctant to go above £20m and the two clubs are so far apart on valuation that they may now pull out of the deal.
Longstaff, 21, is desperate to move to Old Trafford after his representatives shelved contract negotiations with Newcastle in March, but has been left in the dark as talks continue.
United are also struggling to agree a fee with Leicester for Harry Maguire and manager Ole Gunnar Solskjaer warned on the club's tour of Australia that they will not pay over the odds for players.
'We're still in the market looking to do some business but we're patient,' said Solskjaer. 'We don't have to overpay. You have to have the right players at the right price.'
Solskjaer put his players through a double training session for the second time in three days at the WACA on Thursday with the second workout taking place in front of 12,000 fans.
Not everyone in Perth was supporting United, however, and a banner was flown over the famous cricket venue that read 'Liverpool FC 6 x European champions'.
Romelu Lukaku and Anthony Martial missed the session but United insisted that both players trained in the gym even though further talks took place with Inter Milan in London on Wednesday over a deal for Lukaku.
United's only new signings so far remain Wan-Bissaka and Daniel James, and both players had to sing in front of their new teammates along with Scott McTominay as part of an initiation ceremony during a dinner in Perth on Wednesday.