So:
Dear Lennart,
We appreciate your response. We know that we have confirmed that we were able to verify the lower rate that you were claiming and informed you that you will qualify to the Guarantee.
We have gone through sending our request for rate adjustment to the hotel. However, we received a communication from them telling us that the package included in the room on both sites (IHG and booking.com) are not comparable.
[Lange Erklärung mit Screenshot und T&C-Zitat]
Given this above reason, we would like to inform you that we will not be able to extend the guarantee for this particular claim. We know that this will be frustrating on your part but please be informed that the outcome of this validation we made was attained from an appropriate premise. As long as all of the terms of the guarantee have been met, claim will be valid and free night will be honored.
Again, we apologize for all the inconvenience as a result of this decision.
Should you have further questions regarding the specific terms and conditions of the program you may visit
Book With Us Advantage | Best Price Guarantee or First Night Free | IHG
We look forward to accommodating your upcoming reservation. If you need further assistance, please feel free to contact us.
Sincerely"