A billionaire oil tycoon, who just last month agreed to pay $1 billion after a highly contentious divorce, now says it's no longer fair to ask him to pay out the big bucks because his wallet has been hit so hard by falling oil prices.

Harold Hamm this month filed an appeal in Oklahoma saying his personal fortune has taken a hit.

Hamm's oil business Continental Resources (CLR) was worth big bucks during the 26-year marriage because of rising oil prices but now, with oil at about $60 a barrel (it was over $100 at the beginning of the year), the company's value has taken a hit, along with his wealth, he says. As of earlier this year, Hamm was worth over $20 billion, according to Wealth-X, a wealth tracking company.

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Hamm holds a 68% stake in Continental, which he owned before his marriage. However, ex-wife Sue Ann Arnall, an economist and a lawyer, contended during divorce proceedings, and in her own appeal filed this month, that the fortune was due to both her work and his. She says she's entitled to more.

"As Sue Ann has stated previously, 6% is not a fair proportion of the wealth the couple accumulated during their marriage, and she trusts that a more equitable division of the marital estate will result from this appeal," her attorney told CNNMoney in an email.

Still, both appeals could just be a negotiation tactic, said attorney Michael Stutman, who has practiced family law in the state of Oklahoma and is now president of the American Academy of Matrimonial Lawyers in New York.

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"I wonder how dedicated [Hamm] is to the appeal or whether or not he has cross-appealed for some strategic value," Stutman said. "Ultimately I think Mr. Hamm got quite a good deal." He said he thought it was unlikely the appeals court would be convinced by his argument to pay less.

Still, this could open up more negotiation about how Hamm will pay what the court ordered.

He was supposed to pay a third of the $995.5 million owed by the end of the year, and was on a payment plan of at least $7 million per month until the remaining $650 million was paid in full.

It could take another year for the case to wind its way through the court, giving plenty of time for either party to admit defeat -- or for Hamm's wallet to fatten again.

"I wonder if Mr. Hamm will be complaining when the price of oil resuscitates to $120 a barrel," Stutman said.

Whether or not oil prices will surge again is an open question, with some saying we will never see prices at $100 a barrel ever again.

Steve Hargreaves contributed reporting.