https://www.capology.com/player/serhou-guirassy-35136/
This is good for Dortmund. Because this is potentially a big investment that they have made. They have signed him for only 17.5 million euros. If he maintains his scoring productivity here as well, I see no reason for him to increase his market value from 40 million to a much higher level. In the future in case of a bigger team starts to be interested, Dortmund might be able to sell him for at least 60-70 million in my opinion.
In the meantime, I believe Guirassy will play an important role for Dortmund to be competitive in the Bundesliga.
Guirassy was the main reason for Stuttgart to have good performance in the last season of the Bundesliga and they won many games because of this player. Guirassy was expensive when he joined Dortomundand they spent too much money for this player considering this performance but I'm sure as you said Guirassy can improve and have a better performance in Dortmund.
They can use their business and put this player for sale for a huge amount of money just like they sold some other players like Haaland before to the rich teams.
However, while an official offer has yet to be submitted by Dortmund, Bayern Munich is also said to be interested in the Guinea international. Dortmund needs to complete this task quickly.