I'm not at all sure what you are trying to do there - your comparison will always suceed because the two strings will never match.
But if what you are trying to do is convert a numeric value to a NVARCHAR then it's trivial:
select Kategorie, modell, bezeichnung,
case
when '{state:RepEuro}' != 'Smart Repair' then convert(nvarchar(20),(preis),0)
else '140'
end AS Preis
from tblminderwerte
where id = '{state:MinderwertSelect}'
But I'm a little worried that you are showing code from your app, and there is parameter substitution going on, replacing
{state:RepEuro}
with the variable content in your app.
If so, then don't do it like that! Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.
When you concatenate strings, you cause problems because SQL receives commands like:
SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'
The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:
SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;
Which SQL sees as three separate commands:
SELECT * FROM MyTable WHERE StreetAddress = 'x';
A perfectly valid SELECT
DROP TABLE MyTable;
A perfectly valid "delete the table" command
And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.
So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?