According to Phil Kirkbride of the Liverpool Echo, the Toffees have set the nine-figure sum as his price tag to ward off potential suitors, who will be monitoring the striker's situation after he refused to sign a new contract with Everton.
Per the Daily Mail's Chris Wheeler, Lukaku is one of United's top targets as they seek to recruit a centre-forward this summer.
The 23-year-old has enjoyed a prolific campaign, racking up 24 goals and six assists in the Premier League.
Unsurprisingly, the Belgian is among the most clinical forwards in the division when it comes to taking chances, per Squawka Football:
Despite the addition of Zlatan Ibrahimovic last summer and the Swede's excellent form in front of goal, the Red Devils haven't been that much more potent than they were last season under Louis van Gaal.
With his pace, power and proven track record of scoring in the Premier League, Lukaku is a strong candidate to improve United's forward line.
However, while he's an outstanding young player, there are some concerns United may have when splashing out £100 million.
Bleacher Report UK's own Sam Tighe is concerned with his consistency:
His passing is also an issue—while he can be creative going forward and has forged 42 chances for his team-mates this season, according to Squawka, he has completed just 66 per cent of his attempted passes.
With the amount of money on the line, United will need to weigh up if he's worth that sum given the deficiencies in his game and whether or not they can be improved upon.
The 19-year-old has enjoyed a sensational first year with BVB after joining from Rennes last summer, racking up eight goals and 17 assists in all competitions.
Dembele has spent time on both flanks this season as well as playing centrally.
However, he also has areas of his game still in need of development, as noted by ESPN FC's Stefan Buczko during Dortmund's recent 4-1 defeat to Bayern Munich:
The Frenchman has incredible potential and could add another exciting forward player to United's squad alongside the likes of Henrikh Mkhitaryan and Anthony Martial, but the Red Devils have a greater need for a striker.
What's more, Dortmund will likely resist any attempts to recruit him this summer, and as a regular in their side, he has little reason to leave right now.