The Blues are, however, holding out for €7m (£6m/$8m), with Celta Vigo having previously been interested prior to their signings of Joseph Aidoo from Genk and Jorge Saenz on loan from Valencia.
Former Southampton manager Mauricio Pellegrino built up a strong rapport with the centre-back at Leganes last season and is hoping to have him back for the 2019-20 campaign.
West Ham and Brighton are reportedly targeting Youssef En-Nesyri from the Liga club, which could help raise further funds for a deal for the Nigerian.
En-Nesyri is Leganes's record signing at €6m (£5m/$6.5m) and the Madrid club are hopeful that they will not have to surpass that figure to sign Omeruo.
The Blues defender has spoken of his desire to establish himself at a club permanently after moving on loan several times to clubs including ADO Den Haag, Middlesbrough and a further three sides in Turkey.
"I think now’s the time for me to leave, to find a place, to be my own man," he said. "I’m married with a baby now, so I need stability. Chelsea have done amazingly for me, and for my family as well, but it’s time for me to reach my potential and get where I think I deserve to be."
Chelsea have sold a whole host of loanees this summer as they react to pressure from FIFA and UEFA, who are keen to enforce new regulations to cap the numbers that clubs send out on temporary deals.
Kasey Palmer has joined Tomas Kalas and Jay Dasilva in leaving on permanent deals to Bristol City for a combined fee of close to £15m ($18m). Torino, meanwhile, activated Ola Aina's redemption clause and he joined the Serie A side in a £9m ($11m) deal.
Atletico Madrid, meanwhile, have agreed a deal to sign Alvaro Morata for €65m (£59m/$72m) at the end of the 2019-20 season.
There could also be further moves for Tiemoue Bakayoko, Kenedy and Davide Zappacosta before the end of the transfer window as Chelsea contend with foreign player limits in the Premier League and Champions League.
Huddersfield Town have also enquired about Trevoh Chalobah, who is keen to leave the west London club this summer, with the combined sales from the loan group in three consecutive transfer windows likely to vastly exceed £100m ($121m).