contribute page for membership admin use only
If you set your link to the contribution page so that it has the following at the end of the url - &amp;cid=0 - then you should see the page load with no contact linked to it. And if you set the cid to an existing contact id, then people with permission would see the form loaded with that contacts data in place. EG

http://d46.demo.civicrm.org/civicrm/contribute/transact?reset=1&amp;id=1&amp;cid=0 should load with no contact

http://d46.demo.civicrm.org/civicrm/contribute/transact?reset=1&amp;id=1&amp;cid=203 should load with contact 203

Does that help you when combined with the right permissions?