From microsoft and i agree, you need the correct version from the gac and reference it into your project. Once you add the reference you need to set "copy to local" as false ;)
this will work and what i would do if i ran into any dll issue!
step by step:
To copy the dll, goto start - select or search for "run" and copy past this code:
%windir%\assembly\GAC_64\Microsoft.Sharepoint.Sandbox\14.0.0.0__71e9bce111e9429c

add reference to your project, right click on the reference Microsoft.SharePoint.Sandbox.dll and click on "properties" set copy to local = false

build and run project, should work fine :)
http://blogs.technet.com/b/sharepointdevelopersupport/archive/2012/07/24/building-silverlight-solution-with-ria-service-enabled-fails-on-sharepoint-2010-sp1-environments.aspx