Charlize Theron's Pinkberry Tab -- First, She BAILS ... Then Returns With $100

3:15 PM PT -- We just uncovered the most crucial part of this story ... Charlize's fro-yo order! Here it is: chocolate hazelnut with chocolate shavings. Seems a little redundant, especially for $100, but whatever.

Charlize Theron had a Pinkberry worker thinking she got screwed -- the Oscar winner promised to come back to pay her bill, and even though it seemed touch and go for a while ... Charlize eventually came through with a HUGE tip.

Here's what went down -- an employee at Pinkberry in Hollywood says Charlize came in Tuesday afternoon, but when it came time to pay her $3.75 tab ... she plead poverty, saying she didn't have money on her.

The employee, Keila, says Charlize promised to come back, but more than an hour passed before Charlize returned (with Sean Penn, no less).

Watch the video ... Charlize plays it cool on the way out, but right after that Keila showed us why it was worth the wait -- CT left a Benjamin for the $3.75 bill!

Charlize ... definitely not a monster.