Princess Basmah bint Saud bin Abdulaziz Al Saud, 57, a businesswoman, rights activist and member of the royal family, went missing in March 2019 along with her adult daughter Souhoud Al Sharif.
The comment contrasts with remarks made by Prince Mohammed in 2017, after becoming crown prince, in which he called the supreme leader of Iran "the new Hitler of the Middle East."
The decree was never carried on state media nor published in the official gazette as would be normal practice.
Aides said the Crown Prince would prefer to save such a deal for the incoming administration.
King Salman calls for world to stop appeasing Iran, reaffirms support for Arab Peace Plan between Israel and Palestinians.
The 56-year-old princess cited "very critical" health issues in her plea.
Saudi Arabia detained the king's bother, Prince Ahmed bin Abdulaziz, and the king's nephew, Mohammed bin Nayef.
MbS is spearheading a massive transformation of Saudi Arabia to move past being an oil-dependent economy, to move toward a moderate Islam and into a peaceful and technologically advanced future.
Under Vision 2030 plan, naturalization to be available to scientists, senior specialists
Rumors are flying saying that the prince has been giving Lohan lavish gifts and flying her around in his private jet.