It seems Dortmund is not sure about hiring Serhou Guirassy anymore. Serhou Guirassy had an amazing performance during last season in Stuttgart and he was an effective player for this team. This made Dortmund think about hiring this player. But in the medical tests before signing the contract, the situation of Guirassy was not good enough and that's why there is going to be a delay.
Dortmund is ready to pay 18 million euros for Guirassy.
I have also come here to talk about it. Dortmund must be really hesitant with transferring him after detecting this injury problem of him.

Guirassy's injury history so far is like this:
https://www.transfermarkt.com/serhou-guirassy/verletzungen/spieler/270541I understand Dortmund's worries but he is a great striker at the same time. He finished last season with 28 goals in the Bundesliga you know. This was the first ever time we saw him scoring this many goals in a season. While he is having his most productive times, it must be hard for Dortmund to make a negative decision on his transfer. I hope they get him anyway.