I had a casual acquaintance who had a few hundred million dollars managed by a large US bank.
His son--who only had single-digit millions--wanted to buy a house, so the dad talked to the bank and they arranged a zero-interest mortgage for him. They were happy to pass on what they might have earned from the mortgage interest (a few hundred thousand dollars, I'd guess) to make this guy a happy client.
Are you sure it was a real mortgage? Many of these stupidly-rich-person ‘mortgages’ are actually loans against collateral (and can go interestingly wrong if the value of the collateral drops rapidly).
His son--who only had single-digit millions--wanted to buy a house, so the dad talked to the bank and they arranged a zero-interest mortgage for him. They were happy to pass on what they might have earned from the mortgage interest (a few hundred thousand dollars, I'd guess) to make this guy a happy client.