Cannot Attach Database to SQL Server Management Studio 2008 R2
Something seems not quite right with your question.

SQL 2008 is version 655
SQL 2008R2 is version 661

I would expect that you would only see that error if you are attempting to attach a 2008R2 DB to 2008, not vice versa. 

As soon as you attach a DB to 2008 R2, the version is changed to 661 and can no longer be attached to a 2008 server.