“They are demanding advance payment as Ukraine tends to be late on its bills.”
That is a lie and the further dissemination of Russian false propaganda. Russia has been overcharging the Ukraine and failed to fulfill its contractual obligation to make the required deliveries called for in the contract. Russia owes the Ukraine upwards of $15 billion in overpayments, and not the other way around.
OK, I'm interested.
Is there a non-Ukrainian source for that contention?