Wallpaper paste as a carpet adhesive - inexpensive & ecologically sensible
Special adhesives for carpets are expensive and keep the knives so bombproof that a replacement is often only possible with damage to the underside of the expensive knitted fabric. If you have to move, this is often a total loss, although the knitted fabric, e.g. Cropped in a smaller room would have been usable. Especially with a fairly new purchase that is annoying.
Wallpaper paste performs well in this case. He fixed the flooring sufficient, but you can remove the coating again. It's easy with a steam duck. Now clean the carpet and he is allowed to go to the other apartment.