Hi Martin,
What I would do is load all of your list items into the data connection behind the dropdown (I assume your values are from an external data source) and then use the Filter Data option to put a filter the data source entries as follows - my:KostenstellenIdsToShow contains d:ID. This should cause the dropdown to display only the values for which the ListItem ID is contained in your KostenstellenIdsToShow field.
Let me know if that works for you or if you need more detailed steps.
EDIT: just realized this won't work! because if 54 is in your field with Ids, but 4 isn't, it will still think that 4 is in there! Well here's a possible hack: is there any way that you can surround the entire string of Ids with semi colons? For example instead of:
3;54;2;6;4
could you make it:
;3;54;2;6;4;
That way every Id number is preceded by a semi colon and followed by a semi colon, and then your filter can be:
my:KostenstellenIdsToShow contains concat(";", d:ID, ";")
Which would prevent confusion between 4 and 54, for example.