Bharti Airtel fell 3 percent on concerns India's largest mobile operator might need to sweeten its offer if merger talks with South Africa's MTN Group are extended, as widely expected.
"There could be niggling concerns on whether the extension is just to tie-up the remaining loose ends or to thrash out the financial details to garner support of MTN shareholders," V.K. Sharma, head of research at Anagram Stock Broking, said on Friday.
Bharti and MTN agreed in May to hold exclusive talks till July 31 on a deal that could create the world's No. 3 wireless firm with more than 200 million subscribers.
Last week, sources told Reuters the talks would be extended by at least two to three weeks.
Bharti officials were not available for comment on Friday and did not respond to emails from Reuters.
By 0850 GMT, Bharti shares were the worst performer in a a Mumbai market up 1.5 percent.
Market talk of Bharti sweetening its terms has hit its stock ever since MTN's top shareholder, South Africa's state pension fund Public Investment Corp, said in June it believed there was room for improvement on the price.
Bharti shares have fallen nearly 4 percent since the deal was announced in May versus a 13 percent rise in the benchmark index.
Under the complex cash and stock deal, MTN and its shareholders would take 36 percent economic interest in Bharti Airtel and the Indian firm would end up with 49 percent of MTN.