Manchester United are in discussions with Sporting Lisbon in an attempt to conclude Marcos Rojo's proposed move to Old Trafford.
Sporting have rejected United's £15.9million bid for the 24-year-old.
A complicated contractual situation may turn into a legal row, with Rojo's advisors threatening action.
But with the start of the Premier League season approaching, United are attempting to resolve the issue through negotiation.
Rojo, who played for Argentina in the World Cup final, is training on his own after Sporting suspended him.
His representatives, Doyen Sports, believe they are owed 75% of the fee from any rejected bid.
It has said it will take "all necessary measures to defend its rights".
The potential for a long legal case is not something that suits United, who will definitely be without Luke Shaw for Saturday's Premier League game with Swansea and also have an injury doubt over Jonny Evans.
Van Gaal missed out on Thomas Vermaelen last week after the Belgian opted to join Barcelona from Arsenal.
Rojo played at left-back in six of Argentina's seven games during their World Cup campaign, including the final, when they lost 1-0 to Germany after extra-time.
He also scored in their 3-2 group game win against Nigeria in Porto Alegre.
Southampton have also expressed an interest in signing the defender.