Nice have threatened to put Marseille's participation in the UEFA Champions League in doubt.
The Ligue 1 outfit are furious that the reigning French champions have yet to make the first payment on their €15million capture of Loic Remy.
Marseille snapped up the France international last month, but saw the deal delayed by a previously unidentified heart condition.
Remy has since been given the all-clear, but Marseille are keen to restructure the original agreement.
Nice are not so willing to open talks again and have vowed to report their rivals to Uefa should their demands fail to be met.
"They want to give us a bit more on the re-sale (if Marseille sell Remy to another club) but reduce the bonuses (including payments related to Marseille's Champions League involvement)," managing director Patrick Governatori told Nice-Matin.
"We don't agree. We've refused their proposal. For us it is null and void. If Marseille don't pay us on Monday, we're ready to go to Urfa. It could cost them their participation in the Champions League."
Marseille, though, are refusing to budge and president Jean-Claude Dassier has warned Nice that they should get their own house in order before threatening others.
"They can say what they like, they can do what they like, we're sticking to our position," he told the website of regional newspaper La Provence.
"We'll look at it calmly. They shouldn't be complaining anyway. Their words are one thing, the facts are another. When we signed their player, we did the job that they hadn't done.
"We've made them a proposal and if they sign it they'll get their money. And if we have to explain ourselves to the authorities, we'll explain ourselves.
"OM aren't the guilty party, we're the victim. They can threaten us if they like, but they'd be better off improving their medical staff!"
Meanwhile, Nice have snapped up Malian midfielder Abdou Traore on a season-long loan.
The 22-year-old arrives from 2009 champions Bordeaux, with whom he progressed through the youth ranks to become a first-team contender.