The crowns debt to the iron bank
Would the new administration have to pay for the golden army fiasco that the previous payed for
shareThey said the Golden Company had already been paid for.
That dude from the Iron Bank said they would lend the money if Cersei wanted, so maybe. But presumably there was not only enough Tyrell gold left to pay off their debt, but hire the Golden Company as well. Since Cersei apparently had a wagons full of gold to offer people. That's absurd, but the sum they apparently owed the Iron Bank was ludicrous in the show.
As to how they have any money left in the show after the city burned, one can only assume that they managed to salvage the treasury and whatever Cersei had left and added whatever Dany brought over from Meereen. Which you could fanwank is a lot of money since essentially all the treasure the Dothraki leaders had in Vaes Dothrak was hers and all the wealth in the slave cities had been seized.