Watford are reported to have accepted a bid for their French-born Algerian international, Hameur Bouazza.
The 22-year-old has been looking for a move back to the Premier League following Watford's relegation last term.
The talented forward has been linked to a number of clubs, but Fulham have always been in pole position.
Bouazza is now discussing personal terms with The Cottagers, and they are expected to announce his arrival within the next 24 hours.
Boothroyd said last week he would be looking for a figure of £5m for the Academy graduate and he is under no pressure to sell, but the Hornets will find any offer in excess of £4m too tempting to turn down, particularly for a player who has made it clear he no longer wants to play for the club.
The offer represents an astonishing amount of money for the French-born player who was not even in Boothroyd's first XI for the first game of last season and one who spent time on loan at Swindon during the manager's first season in charge.
But he shot to prominence following a sparkling end to last season that yielded memorable goals against Plymouth Argyle, Manchester United and Portsmouth.
Boothroyd has tried to convince the winger his future will be better served by remaining at Vicarage Road, but is now expected to grant the restless Algerian international his wish.
Bouazza was not in the squad to face MK Dons on last Wednesday and appears to have played his last game for the club.
Should the player agree personal terms at Craven Cottage, he will become the second most expensive player sold in Watford's history.
A move to Fulham will see Bouazza go head-to-head with former Hornet Lee Cook for the left-wing spot and reunite him with Fulham coach and former Watford boss Ray Lewington.
"We've had a multi-million bound bid from a Premiership club and that should be resolved within the next two to three days," said the manager.
"I don't want to reveal how much or who it's from as that's not right. I think it's important this gets resolved. We've come a very long way as a club and it's a new situation where our players are coveted.
"But these players are contracted to Watford and that's how it will stay until I decide otherwise."