Robert Rankin, a seasoned investment banker, probably landed the most unconventional assignment of the year: draft the prenuptial agreement for a billionaire casino magnate engaged to Mariah Carey.

Robert Rankin took hire as personal investment manager of James Packer two years ago. As such, he became chairman of Packer's $10-billion casino resort corporation and manager of the billionaire's $5 billion private investment estate.

It's not unusual for investment bankers to go in-house with a major client, but perhaps less natural is how Rankin's investment banking expertise was used: earlier this year, he was pressed into service to draft a prenuptial agreement – prenup – for the billionaire’s marriage to songstress Mariah Carey, herself worth more than $500 million.

Prenups aren’t unusual in the world of the super-rich and, in the case of twice-married Packer, certainly advisable. But the terms of the Packer-Carey agreement, as reported by celebrity gossip website TMZ, are reminiscent of ruthless corporate takeover battles.

$6 Million for Every Married Year

In case of a split, Carey reportedly was to have received $6 million for every year of marriage to Packer, capped at $30 million – a sum she apparently rejected as «tacky and insulting».

Gifts and private jet travel were also strictly regulated under Rankin’s draft. Except for engagement, birthday and anniversary gifts, Carey wouldn’t have been able to keep tokens for «personal adornment» like jewelry, unless Packer gave her a written note. Who says romance is dead?

Carey and her children from a prior marriage were allowed to use Packer’s private jets – he owns three – for their own travel, as long as he approved.

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The 51-year-old Rankin (pictured left) must have been irked to see his years of boardroom experience used to haggle the price of romance, and probably more so to see his legal efforts for naught – Packer and Carey have since broken off their engagement.

$10 Million Engagement Ring

The now-moot agreement underscores the delicacies of dealing with the super-rich, in particular if personal affairs and business are handled out of the same office.

As a successful recording artist, Carey is independently wealthy herself, but marriage to Packer would have propelled her into the rarified stratosphere of the world's ultra-wealthy.

Rankin spent 19 years at UBS including as head of the Swiss firm's investment banking activities in Asia-Pacific until 2009, when Deutsche Bank poached him for their top Asia job. He left banking altogether two years ago to work directly for Packer, with whom UBS has long maintained close ties to and carried out countless deals for.

Breakups are always hard, but Carey has some comfort: she is reportedly keeping an engagement ring from Packer worth $10 million.