Nigerian Communication Commission (NCC) has pushed back the deadline for payment of the $ billion fine it slapped on Africa’s largest telecom operator, MTN Group, unit in the country until negotiations are concluded, the mobile service firm said in a statement.

MTN was slapped with the largest fine any African company has ever faced  for allowing 5.1 million subscribers in the West African nation to continue using unregistered SIM cards on its network after a deactivation deadline expired.

The firm’s shares have shed nearly a fifth of their value since the fine was announce on Oct. 26 and prompted the resignation of the its chief executive.

The fine was due on Nov. 17.

“Shareholders are advised that the Nigerian authorities have, without prejudice, agreed that the imposed fine will not be payable until the negotiations have been concluded,” MTN said in a statement on Monday.

MTN said former chief executive and current executive chairman, Phuthuma Nhleko, had personally met with NCC officials to discuss the fine. The negotiations include matters of non-compliance and the remedial measures that may be adopted to solve the matter.

“The fact that they have pushed back the deadline shows that they are seriously negotiating and perhaps we could see a reprieve,” Sibonginkosi Nyanga, an analyst at Imara SP Reid’s in Johannesburg, told Reuters.