He’s more so saying it’s more money in the states than in the UK not that they are really capped
Thing is he can maybe make more money in the US against Joshua than in the UK (not sure at this point, w no Joshua debut in the US yet) but he probably can't make more against Fury in the US than against Joshua either in the UK or US. And with Joshua being the bigger draw all he can do is tell them that he thinks they would get more in the US but if they would still want it to be in the UK he's still the one who would have to budge.
I think if they fight in the UK first but a contract states that either half has the right to call for an immediate rematch which would take place in the US regardless of the result would be fair enough. That way the fight would probably go to the US too except if one of them would be destroyed so bad that he would even turn down the millions at an immediate rematch cause he just won't want it no more...