The World Boxing Council (WBC) had ordered heavyweight champion Deontay Wilder to arrange a rematch with Tyson Fury by February 5.
The American retained his WBC belt with a controversial split-decision draw against Fury in December.
The WBC has told its promoters to agree to a rematch by February 5 or they'll split the purse bid 60-40 in favour of Wilder, "considering the market value of Fury", as opposed to the usual 70-30 in favour of the titleholder.
"The WBC is hereby notifying both camps that the free negotiation period is opened and if there is no agreement between the parties, a purse bid will be conducted by the WBC on Tuesday, February 5," the WBC said on Thursday (NZ time).
"The WBC has modified the 70-30 split and has confirmed a 60-40 split in favour of the champion Wilder, considering the market value of Fury."
When the two fighters battled last month in Los Angeles, Wilder scored two knockdowns, but Fury beat the 10-count both times and after the bout, both endorsed the idea of a rematch.
The rematch order leaves IBF, WBA and WBO champion Anthony Joshua still looking for an opponent for his Wembley bout in April, with fellow Brit Dillian Whyte and American Jarrell Miller the two likely opponents.
Joshua has indicated that Wilder is his top target for the April 14 bout.