After fighting ended earlier in the month, a Fatah al-Islam prisoner identified a body as the leader of the group. Abssi's wife and family members also positively identified the body.
The Lebanese army ran DNA tests after taking blood from his relatives.
A statement from the public prosecutor's office said the DNA tests showed the body's sample did not match those from Abssi's children and brother.
"Thus, the body preserved in the morgue of Tripoli Governmental Hospital does not belong to ... Shaker al-Abssi," the statement signed by the public prosecutor said.
The statement quoted the confession of a captured Yemeni fighter who said he escaped the camp on September 1 with Abssi and others.
The Yemeni man, reportedly captured after losing contact with Abssi, was quoted as saying: "Shaker al-Abssi was in good condition and was wearing a suicide belt and carrying a Kalashnikov rifle, magazines and hand grenades."
Rebuilding the camp
Fouad Siniora, Lebanon's prime minister, called a donor's conference to seek funds to reconstruct Nahr al-Bared camp.
Unrwa, the UN agency which cares for Palestinian refugees, said it needed $55m "to fund the first year of rehabilitation and emergency assistance to refugees from the Nahr al-Bared refugee camp".
Hoda Elturk, Unrwa spokeswoman, said: "We are very happy with the response of the donors and we are expecting more pledges to come."
Most of the refugees fled to the nearby Beddawi camp in the early days of the fighting and are not expected to return to Nahr al-Bared soon. Most of its buildings are in ruins and the camp is littered with landmines.
However, the $20m pledged fell well short of estimates of what was needed to cover the costs of adequate relief for the refugees, the reconstruction of the camp and help for nearby municipalities affected by the fighting.
The Lebanese government estimated the total required at $382m.
The United States led pledges at the donor conference, with $10 million. Germany offered $5.52m, Norway $1.81m and Italy about $2.74m.
Most other countries refrained from setting specific amounts. Saudi Arabia had pledged $12m during the fighting and the United Arab Emirates earlier pledged $5m in aid.