A Syrian refugee sold a kidney to pay for his passage to Europe, a Serbian aid worker said.
The organ was removed in a backstreet and dangerous operation in Lebanon and the victim went 20 days without proper medical care before his discovery last week by an anti-trafficking organisation in Serbia.
He was offered treatment but opted to travel on with only painkillers, the Belgrade-based Atina anti-trafficking organisation said.
Aid worker Naser Amro said: "He sold his kidney to get money to travel. He said he had pain and problems so we intervened to get him to medical help.
"After talking with the individual he told us his story."
Mr Amro said the victim was not in a life-threatening condition.
"It was a 20-day-old operation and it was not done the proper way of course. He was suffering from pain and he was provided with painkillers.
"He did not want to stay, he wanted to continue his journey, he was already on the border with Croatia."
Mr Amro said it was the worst case of human trafficking Atina had discovered recently.
The United Nations High Commission for Refugees (UNHCR) which is co-ordinating rescue efforts is aware of the case, another aid organisation confirmed.
Atina said It had no evidence of traffickers operating in Serbia, they congregate around the treacherous sea crossing from Turkey to Greece.
But when the victims reached Serbia en route to western Europe they could be identified through bruising from mistreatment or good communication and translators of Arabic and Farsi.
Mr Amro said: "Two weeks ago two Afghan girls were sexually exploited. A man from the UNHCR provided them with support."
A family whose daughter aged two-and-a-half drowned in the Aegean was in a dangerous boat with smugglers who drove their victims on with threats, the aid worker said
"There is a point in Turkey which the refugees reach and there is no going back.
"If they want to step back and not travel at night on water they cannot."
Faisal Khalib, 32, was on a bus from the Serbian border towards a registration centre. The recently qualified lawyer from Damascus was bringing his young nephew to his sister in Munich.
Smugglers forced him to captain the dinghy from Turkey to Greece.
"I cannot even cycle a bicycle but I went to sea as a pilot."