How do I install "OutDir" on the command line with Mono xbuild.exe?

I am trying to build a C # solution in msysgit (Windows) using the same build command as with msbuild:

"C:/Windows/Microsoft.NET/Framework/v4.0.30319/msbuild.exe" /p:Configuration=Debug /p:OutDir="c:\projects\proudly\build" "src/Proudly.Identity.sln"

With msbuild, the build output goes to my folder /build

and all is well. Now when I run the same command with Mono xbuild like so:

"C:/Program Files (x86)/Mono/lib/mono/xbuild/12.0/bin/xbuild.exe" /p:Configuration=Debug /p:OutDir="c:\projects\proudly\build" "src/Proudly.Identity.sln"

... and my build output goes /bin/Debug

to a per project folder instead of my build folder.

I was under the impression that xbuild can be used in the same way as msbuild. Any hint what I am doing wrong?