The Mexican is supposed to get around $9million (£7.5m) to fight Joshua in December, but apparently wants “significantly” more, before he agrees to the bout.
The rematch for the IBF, WBA and WBO titles is set to take place in Diriyah, Saudi Arabia on December 7.
Ruiz Jr recorded a shock win over Joshua in June, after he replaced Jarrell Miller who failed a drugs test.
He dropped the Brit four times, before the referee called off the
Joshua quickly activated a rematch clause in his contract after the fight.