Is the approval to retain a resected part of the intestine—containing a rare cancerous tumor—for research and study purposes, an act for which its doer will be rewarded, or must it be buried?

The excised part of the intestine does not have to be buried, but it is commendable to do so. Your agreement to keep it for studies is hoped to be rewarded, because it assists in research that may lead to the discovery of medicines beneficial in treating or preventing the disease, and "every act of kindness is a charity."