The key points are age and potential. Leverkausan must see a player in Frimpong that they can develop get 2/3years playing time out of him then sell on at a profit. It's probably a 4/5 yr plan to sell him aged 24/25 to a bigger club.
Our problem is a few of our better players, Tav, Borna, are at the age when that isn't feasible so won't realise inflated fees. Teams with short term needs are their outcome.
Our sellable assets are the young ones like Kent, Morelos, Hagi. This is where we need to maximise any transfer fees.