The way from SQL 2008 to SQL Server 2008 R2 is a major upgrade (like from 2000 to 2005) thus you cannot use the new file format in the old SQL version.
Even when the software does not need the R2 version the database needs it and that's the problem.
You can try a Bulk copy utility to export the database contents and import it into the older version. This should be database independent. You can also try to link the old server from the R2 version and port all your tables.
