The two clubs are involved in an advanced talks over the midfielder.
And the sticking point is not understood to be the fee, but the terms of the deal.
Newcastle – who travel to Doncaster Rovers tonight – want a large part of the fee to be paid up front, while Liverpool have proposed staggered payments.
However, the issue could yet be resolved in time for Wijnaldum to cross the Atlantic with Jurgen Klopp’s squad later this week.
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Liverpool, set to embark on a pre-season tour of the USA, are yet to lodge a formal bid for Wijnaldum, who is keen to move to Anfield.
But that could change over the next 24 hours.
Wijnaldum, signed from PSV Eindhoven last summer for £14.5million, came off the bench and scored in Saturday’s 6-0 win over Bohemian in Dublin.
The 25-year-old was allowed to fly to Holland after the game.
And his 21-minute run out at Dalymount Park could turn out to be his last appearance for the relegated club.
Wijnaldum is not expected to travel to Doncaster with his United team-mates for the club’s fixture at the Keepmoat Stadium.
Newcastle manager Rafa Benitez wants to keep Wijnaldum and Moussa Sissoko, his midfield colleague, at St James’s Park.
But Benitez, having already allowed striker Papiss Cisse to leave, has acknowledged that the club could be prepared to sell more players this summer if the “right offers” came along.
Liverpool’s move for Wijnaldum came as the club upped the stakes in its own bid for Blackburn Rovers defender Grant Hanley.
And the club is still hopeful of signing the 24-year-old, who is also a target for Norwich City.