The transaction, which concluded earlier this year, included a joint bid by TIM, America Movil's Claro and Vivo to acquire Oi's mobile operations at an original price of about 16 billion reais.
But post-deal analysis showed the need for an adjusted closing price, TIM said in a securities filing, citing divergences in working capital, capital expenditure and net adds data that were seen in documents available only after closing.
Under the contract agreements on price adjustments, Oi would need to repay a total 1.75 billion reais if the three buyers succeed in their deal appeal.
In a separate filing, Oi said it strongly disagreed with the request to cut the price by about a fifth, which it dubbed as based on technical and procedural errors, and that it will take measures to contest it.
The company said it might issue a formal disagreement notification within the next 30 days.
The potential post-deal price adjustment led common shares in Oi to plunge roughly 10% in morning trading.
Telefonica Brasil - which operates under the Vivo brand in the South American country - was trading near flat and Telecom Italia's TIM dropped 1.3%.
($1 = 5.2888 reais)
(Reporting by Gabriel Araujo; Editing by Christian Plumb and Jane Merriman)
