If you only need to .erb the sass files to include images, you can use the image-url
helper provided by the sass-rails gem. This way you can keep files as .css.scss and TextMate highlighting will work.
background: #fff image-url("search.png") no-repeat scroll left center;
More info:
http://guides.rubyonrails.org/asset_pipeline.html#css-and-sass
If you have to .erb the sass files then the syntax highlighting in TextMate will only be fixed by adding support for a new language for scss.erb files in a bundle. This would be similar to the HTML (Rails) language in the popular Ruby On Rails bundle by drnic.
https://github.com/drnic/ruby-on-rails-tmbundle/tree/master/Syntaxes