So who pays this? Trump says it won't be us but this clearly implies we broker it.

Well, no one's actually going to pay it, since this is unlikely to ever be implemented.

If it were to actually be implemented, it would be private investment money. This isn't money that gets paid to Iran. Instead, it's an investment fund - private parties (including sovereign wealth funds) agree to provide money to do private infrastructure and redevelopment activity in Iran. Kind of like the whole "Board of Peace" slush fund, where governments had to allegedly pay up in order to join the BoP - but instead, it's not governments themselves but private parties and the sovereign wealth funds.

It's just another instance of Trump misunderstanding the motivation of the Iranians. The intent is to create a vehicle to incentivize both Iran and the U.S.'s allies to reach a Final Deal once the MOU negotiations get started. The pitch to Iran's leaders is obvious - "Give up your nukes, and you all can get rich!" The pitch to the Gulf States (who will have to live with the newly unchained Iran) is equally simple - "You all get a piece of the reconstruction action and a change to increase your influence in Iran."

The problem, though, is that the Iranians aren't motivated by that. It's the same reason the BoP fund failed in Gaza. The leaders of Hamas aren't gangsters pretending to be motivated by ideology. They're true believers. They're not interested in becoming Gazan oligarchs. The same is almost certainly true of Iran. Those leaders aren't going to be much tempted by a $300 billion slush fund they can dip their beaks into. They're true believers.

But, since Trump gave away most of the carrots that were in the JCPOA as a precondition to negotiating, they're not available to be part of an actual nuclear deal. So they have to come up with something.