robots.txt is not your technical solution here.
you should make sure all your internal links refer to the canonical urls.
(such as http://example.com/aboutus.html and http://example.com/details/50.html)
you should implement RewriteRules such that any requests for the dynamic urls are externally redirected to the static urls.
you should also implement RewriteRules so that any requests for the static urls are internally rewritten to the dynamic urls.