Mary-Kate Olsen dumped her hard-partying artist boyfriend after they couldn’t agree on a whopping $900 million prenuptial agreement, say sources.

The 23-year-old star – who with her twin sister Ashley has amassed an astounding billion dollar fortune – decided to call it quits with Nate Lowman in mid-February.

The pretty performer began dating 30-year-old Nate two years ago, but recently she realized the romance had run out of gas, divulged a close source.

"Mary-Kate, who is very much a romantic, knew it was over and made sure she told him so before Valentine’s Day. She tearfully dumped him.

"The relationship began falling apart in November, and it never recovered over the last three months. She’s down in the dumps about it, but doesn’t rule out a reconciliation."

Back in our Nov. 2, 2009, issue, The ENQUIRERr eported that Mary-Kate had wanted to take their relationship to the next step, and because many millions of dollars were at stake, her legal advisers wanted to make sure a prenup was ironed out.

"Following the prenup problem, Mary-Kate gave Nate his walking papers after his partying became too much," the source divulged.

For ALL the details you won’t find anywhere else –pick up the new ENQUIRER – on sale now. PLUS learn WHO M-K’s now getting busy with!