Extracting Text Value in DAX

First, DAX Formatter is your friend.

Second, column references only evaluate to scalars in row context. Measures operate in filter context. You need to wrap every column reference outside of row context in some function.

InRent = IF(
    HASONEVALUE( propuserdefinedvalues'[value] )
    ,CALCULATE(
        VALUES( 'ruhor_views propuserdefinedvalues'[value] )
        ,'ruhor_views propuserdefinedvalues'[userdefinedid] = 56
    )
)

This should do it for you. VALUES() returns the distinct values making up a field (in filter context), and its result can be coerced to a scalar value if there is only one distinct value.

Secondly, for simple predicates, there's no need to use FILTER(), CALCULATE() can take simple literal predicates as direct arguments.

Lastly, the measure I've provided shouldn't throw any syntax errors at you, but you're checking for one value of [value] BEFORE you're applying a filter on [userdefinedid]. Thus, this measure will be blank if the [propid] selected has multiple distinct values of [value] across all [userdefinedid]s.

You'll probably need to change it to something like the following:

InRent = IF(
    CALCULATE(
        DISTINCTCOUNT( 'ruhor_views propuserdefinedvalues'[value] )
        ,'ruhor_views propuserdefinedvalues'[userdefinedid] = 56
    ) <= 1
    ,CALCULATE(
        VALUES( 'ruhor_views propuserdefinedvalues'[value] )
        ,'ruhor_views propuserdefinedvalues'[userdefinedid] = 56
    )
)