The Edinburgh-based oil and gas explorer previously announced its intention to pay that sum to buy a 35% stake in a block 30 kilometres off the coast of Mauritania from Chariot Oil & Gas.
The agreement between Cairn subsidiary Capricorn Mauritania and Chariot Oil & Gas Investments (Mauritania) has now been approved by the Ministry of Hydrocarbons from the African state.
Chariot, which has its registered office in Guernsey, said: "All other approvals and conditions precedent to the transaction have been granted or fulfilled."
Chariot said it has now received payment from Cairn for the cost of obtaining 3D seismic data and some other charges it had incurred. The vast C19 block covers around 12,175 square kilometres with water depths ranging from five metres to more than two kilometres.
Two wells previously drilled in shallow water areas of the C19 block had reservoirs with oil shows which may indicate the presence of oil deposits below the seabed.
A further set of data on the block is expected to be received next month and Chariot said a resource update is likely to be published in the first quarter of next year. It added: "Drilling planning and the evaluation of partnering options will commence thereafter."
The agreement means Chariot has 55% of the block, Cairn 35% and the Mauritanian state-owned SMH retaining 10%.
Other companies operating in the region include Dana Petroleum, Tullow Oil, Petronas and Kosmos.