The 22-year-old centre-back has featured regularly for Blaugrana during pre-season but the Catalan club are willing to cash in on him.
Marlon spent last term on loan at Nice and has recently been the subject of a €12million bid from Serie A outfit Sassuolo, but Schalke are thought to have more cash available at their disposal to land the Brazilian.
He could be the latest defender to leave the club after duo Yerry Mina and Lucas Digne joined Everton.
England’s World Cup centre-back star was widely linked with the Red Devils during the summer transfer window but a move never materialised.
It is now expected the 25-year-old will now be offered increased terms of £75,000-a-week – a huge increase on his current £30k per week wage.
Maguire is contracted to the Foxes until 2022.
Monaco are hoping to sign Porto star Yacine Brahimi, according to France Football.
The French Riviera side are looking to strengthen their attacking options following the departure of Keita Balde to Inter Milan this week.
Algerian international Brahimi has a €60 million (Dhs 249m) release clause, but Monaco hoping to broker a better deal with the 28-year-old’s contract expiring next summer.