It makes no difference which version you have chosen and use. Either the version with the trailing slash or the version without the trailing slash should be indexable. The version that should be indexable should also have a Canonical Tag reference coming from the other version.
Technically speaking, the version http://www.domain.de does not exist. The reason for this is that every request must always be specified in a path. The smallest possible path is specified between two "slashes" "/". Therefore the smallest possible path of a domain is "http://www.domain.de/" (The same applies to https).
For the Google ranking it makes no difference which version is indexed.
Our crawler analyzes the page with a slash at the end of the path, so we automatically add a trailing slash to the domain.