Firas normally rips off their stories from other sources, but I could not find the original story, just a copy in a Fatah forum quoting "Kofia Press" which I am unfamiliar with but looks to be a Fatah website.
While it sounds like a wild rumor, what makes this story so interesting is the amount of specificity: it names the doctor, the victim, the dates, and the place of the alleged crime:
The perpetrator is a doctor named Mohammed Rashid Abdul Hamid Deeb, belonging to Al Qassam Brigades, who lives in the Nasr district, near the Turquoise towers in Gaza City, with identity number 900336215, who was born on September 12, 1974 and works in the Ministry of Health. The victim is mentally handicapped citizen named Anwar Hassouna - known as Anwar Abbas - from Beach Camp he was thirty-eight years old.The article goes on to say that this case indicates that other "natural" deaths in Gaza could have been murders by Hamas.
Anwar was kidnapped on 15.06.2010 and then [Deeb] killed him with an air injection into a vein leading directly to death, for the removal of his kidney, and the victim's body was disposed of in the area of Wadi Gaza.