Run-time Changeable Queries

I need to extract and store a value from a table (or from a MS Access file with OpenDataSource) which is not always the same and it is therefore stored in the @openfile variable. Something like this:
...
declare @standardselect nvarchar(4000)
declare @value int
select @standardSelect='select top 1 @value=val from ' + @openfile
exec (@standardSelect)
...

It obviously doesn't work because the variable @value is not declared within the sql string.
However, since @openfile is always different, I need to pass it through a string and the only way I know is within a variable. If I declare @value inside the @standardselect it is not accessible to the rest of the procedure, which is not acceptable for me.

Any suggestions?