They called him the diamond eye at Dortmund and that's what he's been doing at Stuttgart, too. He started there after they had gotten relegated and they won promotion despite having to extract €55m from the squad in fees. Signing players such as Kalajdzic, Kobel, Endo and Silas (was valued €25m on TM, before he tore his ACL) in 2. Bundesliga. After promotion they had a quiet window, where they mostly made some previous loans permanent and signed Mavropanos for peanuts. But then Corona hit and in the following summers they had to extract €25m, €35m and €30m in net transfer fees from the squad that was newly promoted to begin with. They were cutting it close with relegation a bit, but I think he deserves a lot of credit for creating so much value with his transfers, that they were able to have a competitive and talented squad and adding further promising players, despite being forced to create massive transfer profits each year.
As a parting gift he signed Vagnoman, who will probably fetch them at least a €10m profit once he matures a bit, Ito, who was signed for €400k from J-League and is curently rated €10m on TM and Guyrassy, who saved their necks last year and currently leads the league with 8 goals in 4 games - he got him on a "free" loan with a €9m buy option.
He's reportedly quite opinionated and can be a bit hard to work with, he also strikes me as more of a Klopp fan than Cruyff worshipper but when I heard the news I thought he could still be a great fit: him coming up with bargains from obscure clubs and Ajax/Eredivisie being the perfect incubator to pump up their value. Creating some massive transfer revenues that would eventually also strengthen the (more stable, long term) core of the team.