Add a Inline like this:
LOAD * INLINE [
Then Add a Table with this expression:
if(getfieldselections ( Field) = 'Sales qty', [Sales qty],
if(getfieldselections ( Field) = 'Sales amt', [Sales amt],
There you go
This feature isn't in the newest Qlik Sense version AFAIK. You can't dynamically label the field in a simple way.
What you can do is to label the column as just 'Selected Field' and the user will understand that the field that the column has the same field as he selected in the listbox 'Field'.
Hope this helps you