Recent speculation has linked the Reds with an offer in the region of £7million for the Argentina full-back, but any move appeared to have been ruled out by United boss Sir Alex Ferguson after an initial bid for the player was rejected.
But Benitez has refused to back down in his pursuit of the 29-year-old and has revealed: "We made an offer which they rejected.
"The lawyers are now working on it and so we have to wait. He's a good player."
Ferguson last week poured scorn on any suggestion he might allow the former Paris St Germain player to leave for his club's bitterest rivals.
He said: "I can assure you, Liverpool will not be getting Gabriel Heinze.
"We can put that to bed right now and we have done so. We have had a couple offers for him and we have turned them down."
It later emerged that Heinze had a letter signed by chief executive David Gill promising he could leave if United received a big in excess of £6million - a figure Benitez is thought to have topped.
His representatives were reported to be considering legal action to push through the controversial deal - the first between the two clubs since Phil Chisnall moved to Anfield in 1964 - and it appears the Reds are also now willing to go to court to get their man.