The state-run company has offloaded a 49% stake in Nova Fronteira to local ethanol producer São Martinho.
The original call that drew three offers was declared void.
Work on the Brazilian petrochemical project was stalled in October 2015 after local prosecutors said they were investigating alleged contract irregularities as part of their Lava Jato corruption probe.
The exception to the new 80% policy will be for credit given to municipalities for reconstruction after natural disasters, which will keep its 100% limit.
State governors have met with the finance and energy ministers to study ways of reducing fuel prices.
Apart from the political backlash, Wood Mackenzie identifies further risks associated with the upcoming liberalization of the market.