Three South African mercenaries have pleaded guilty to charges and agreed to testify against Sir Mark Thatcher, who is accused of financing a coup.
Sir Mark is due to reappear in court on 25 November
They admitted involvement in a conspiracy to overthrow Equatorial Guinea's President Teodoro Obiang Nguema.
The plot was foiled in March when scores of suspected mercenaries were arrested in the country and Zimbabwe.
Sir Mark denies any involvement in the coup and is due to reappear in a Cape Town court on 25 November.
The three South Africans - Crause Steyl, Lourens Horn and Harry Carlse - were fined and received suspended sentences after admitting breaking South Africa's Foreign Military Assistance Act.
Lawyer Alwyn Griebenow told the AFP news agency: "We did indeed make a plea bargain with the state.
"They do want us to testify and we said we would be willing to do that."
Steyl, a pilot, was ordered to pay a fine of 200,000 rand (£17,800) or serve 10 years in jail.
Horn and Carlse were fined 75,000 rand (£6,600) or four years in jail after they entered a guilty plea in Pretoria's magistrate court.
"The fines have already been paid and we are on our way home,"
Mr Griebenow said.
The trio also received four-year sentences suspended for five years.
If the men break any mercenary laws in the next five years, they will immediately be sent to jail for four years.
Sir Mark, the son of former prime minister Margaret Thatcher, was arrested in August by South African police and released after posting bail of £167,000, reportedly paid by his 78-year-old mother.
He is accused of helping to fund the purchase of a helicopter, breaching laws banning South African residents from taking part in foreign military action.
His lawyers maintain the funds were an investment in an air ambulance venture for west Africa.
Sir Mark could face 15 years in jail if convicted in South Africa.