As Khaled says, 20m is just a number put out there by his team most likely.
He would never find this in Europe, 12m-15m is a very good estimation for his best case scenario.
Also correct to say he is being underrated. A lot of people calling him average, but the reality is the fullback/wingback position is very limited in global football, and there aren't many top RB/LBs out there full stop let alone top ones for either side alone. The position(s) are sparse at the moment.
He is still a top player at his position and has better athleticism than Balde whilst being slightly better all around.
With that said, given that signing him would likely cost a decent amount, and Balde is still a very solid player with great room for improvement being only 21, we shouldn't consider it at least until we fix our pending problems in other positions like ST, (DM?), and possibly GK as its been time to sign competition for Ter Stegen for a long while now, who is going to be coming off a major injury.