Russian and Ukrainian energy ministers are planning to discuss on Friday in Berlin how Ukraine plans to pay Moscow for natural gas.

Europe’s Energy Commissioner, Guenther Oettinger, who will mediate the talks, said earlier this week that Ukraine must make a down-payment to Russia for the gas and Russia must offer a fair price, in line with market conditions, for further deliveries. The dispute threatens gas supplies to Europe, as Ukraine is the transit nation for half the gas Russia sells to the EU.

The Russian and Ukrainian governments agreed on Monday to study Oettinger’s proposal that Ukraine pay Russia $2 billion by Thursday and another $500 million by June 7.

Alexei Miller, CEO of Russian state-controlled gas exporter Gazprom, said Wednesday that Ukraine owes $3.5 billion for gas and will owe $5.2 billion as of June 7 if it makes no payments until that date. Gazprom had previously threatened to stop supplying the Ukraine with gas if it doesn’t pre-pay for gas by Monday.

Ukraine responded on Wednesday by saying it only owes $2.5 billion due to Russia’s seizure of Crimea.

It is not yet clear whether Russia or Ukraine will accept the EU proposal.