Don't forget the @.

app/views/projects/merge_requests/widget/open/_rebase.html.haml

@@ -21,7 +21,7 @@
     = hidden_field_tag :authenticity_token, form_authenticity_token
     .accept-merge-holder.clearfix.js-toggle-container
       .accept-action
-        = f.button class: "btn btn-reopen js-rebase-button", disabled: !merge_request.approved?, ":disabled" => "disableAcceptance" do
+        = f.button class: "btn btn-reopen js-rebase-button", disabled: !@merge_request.approved?, ":disabled" => "disableAcceptance" do
           Rebase onto #{@merge_request.target_branch}
       .accept-control
         Fast-forward merge is not possible. Rebase the source branch onto the target branch or merge target branch into source branch to allow this merge request to be merged.