Brentford have agreed an initial £12.5m deal with Liverpool to sign Caoimhin Kelleher.
Performance-related add-ons could take the eventual fee to £18m.
The Republic of Ireland international is expected to undergo a medical with Brentford on Monday.
Kelleher is someone Brentford have tracked for a long time and they were interested before they signed Flekken, but Liverpool have only been open to selling him since securing Giorgi Mamardashvili from Valencia last summer.
Brentford have been facing competition from other clubs for Kelleher, 26, including from Leeds and West Ham, but Thomas Frank's side have a strong relationship with Liverpool having secured Sepp van den Berg and Fabio Carvalho from Anfield last summer.
Once the transfer is complete, Mark Flekken can complete a move to Bayer Leverkusen.
Leverkusen and Brentford have agreed a fee in the region of £11m (€13m) inclusive of add-ons for the Netherlands international Flekken.
Sky in Germany are reporting Flekken is ready to undergo a medical with Leverkusen.
If the deal is completed without issue, Flekken will link up with former Manchester United boss Erik ten Hag in Germany.
For an initial net outlay of £1.5m, Brentford are replacing a goalkeeper who is 32 in 12 days for a 26-year-old from Liverpool - a shrewd piece of business from Brentford's director of football Phil Giles and technical director Lee Dykes.
Hallgrimsson wants to see Kelleher move
Republic of Ireland boss Heimir Hallgrimsson spoke openly about wanting Kelleher to leave Anfield in search of regular minutes as a first-choice goalkeeper elsewhere.
Kelleher, capped 22 times for his country already, secured his second Premier League title with the Reds this season but only started 10 games with Alisson ahead of him in the pecking order.
"I just want him to go to a club where he will play on a regular basis," said Hallgrimsson earlier in May.
"He has been playing at the highest level, whether it's the Champions League or the Premier League, and done really well. I just want him to be happy and playing regularly.
"I know he wants to be playing at the highest level. I guess - I don't know for sure - he will be playing at the highest level.
"He has spoken about that himself. I don't mind [where], so long as he's playing on a regular basis because he is too good not to be playing."
Kayode signs for Brentford
Brentford are looking to get their business in the summer window done early after already confirming the signing of Michael Kayode on a five-year deal. The club also have an option to extend his contract by an additional 12 months.
Kayode began a half-season loan with the club from Fiorentina in January, which has now been made permanent until at least 2030.
On signing his new contract, the 20-year-old defender said: "I am so excited to extend my stay at Brentford.
"I have felt so welcome from the first day I arrived at the club, thanks to my team-mates and the care the coaching staff have shown me.
"I've wanted to play in the Premier League since I was a child and it's been so good to do that here. I can't wait for more next season and beyond!"
The first of two summer transfer windows is upon us - and there is no better place than Sky Sports to get all the latest transfer news and rumours.
Use the Sky Sports app and website for all your updates in our dedicated Transfer Centre and Premier League club blogs, plus live Q&As with our reporters throughout the summer.
The Transfer Show returns to Sky Sports News from Sunday June 1 for the start of the pre-Club World Cup window and will then be on every weeknight at 5pm and 7pm until the deadline on Tuesday June 10.
The summer transfer window will open again on Monday June 16 until Monday September 1 - with the deadline brought forward to 7pm this year.
