Prince Harry will receive a huge sum of cash next month, thanks to his late great-grandmother.

Queen Elizabeth, the Queen Mother, put $90 million into a trust fund for her family, and listed as one of the stipulations that Harry have access to his share — a reported £7 million ($12.3 million) — when he turns 40.

That big day is Sept. 15.

“Why 40? To a woman who nearly lived to be 102, one can only imagine that 40 seemed like the perfect age to come into some extra cash — when you’re young enough to appreciate it but also old enough, and hopefully wise enough, not to blow it,” Christopher Andersen, author of The King, recently told Fox News.

“Of course, the princes’ great-granny couldn’t have anticipated all the trouble Harry caused the rest of the family,” he said. “I think she would have cut him out of [the trust] entirely had she known.”

Andersen explained that knowing Prince William would become Prince of Wales after Charles was crowned King of England, Harry’s big brother would inherit the Duchy of Cornwall, which boasts assets of more than $1 billion.

“In divvying things up, the Queen Mother may have left Harry a skosh more,” he said. “In the meantime, the value of the trust has simply increased, Harry’s share along with it.”

Royal podcaster Kinsey Schofield, host of To Di For Daily, pointed out there shouldn’t be any tension expected that Harry will receive more than William given that the Royal Family knew for decades about the arrangement.

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“The Queen Mother knew he would have fewer opportunities and responsibilities. It was a logical and thoughtful decision,” Schofield noted to the outlet.

Prince William is believed to have inherited his share two years ago when he turned 40.

He also received a ceremonial bell concert at Westminster Abbey on the milestone birthday.

However, perhaps the difference in monies is affecting William more than believed as sources close to the Prince and Princess of Wales don’t believe he deserved how much he is set to be paid.

“William’s pretty disgusted that once again Harry’s going to be cashing out and taking millions back to his Montecito home without lifting a finger for it,” an insider told Closer magazine.

“There’s nothing he can do about it, aside from shaking his head, but if it were up to him, Harry would not be getting this payday.”

The source added: “William doesn’t think his brother is in any position to be getting this handout and he’s letting it be known that he thinks it’s a pretty rotten situation.”

William believes Harry is “being rewarded after the terrible treatment he’s shown them all,” the source claimed.

“William and Kate don’t trust Harry or Meghan one bit, so the idea that Harry’s now cashing in on his great-granny’s inheritance leaves a very bitter taste in their mouths.”