Ambar Roman scooped up the memento in the seventh inning of LA's tilt with the Giants at Dodger Stadium -- after Ohtani blasted it some 430 feet from home plate into her seat in the right field bleachers.

It was Shohei's first-ever dinger as a Dodger ... and, initially, Roman was fired up to score the treasure. She was seen on the game's broadcast going crazy -- but just minutes later, she said things turned very stressful.

In a conversation with The Athletic, Roman accused Dodgers officials of separating her from her husband ... before insisting she fork over the ball for just a couple of signed Ohtani hats.

Roman said the team hinted they wouldn't authenticate the ball if she didn't make the trade -- and with no ability to talk things over with her husband ... she says she did the deal after they sweetened the pot a bit with an additional signed bat and ball.

But now, her husband, Alexis Valenzuela, said the whole thing left a bad taste in his and his wife's mouths.

"I was just disappointed that a team that I hold so dear pulled a quote-unquote quick one on us," Valenzuela said.

"We're not trying to extort anyone," he added. "It's not that we're money hungry. It's just that it's a special moment, it's a special ball. I just think it's fair for it to be equally rewarded."

According to a Heritage Auctions expert, the ball is worth in the neighborhood of $100,000. The signed memorabilia Roman received was worth around $4,000, the expert said.