I saw this phrase used in a post by a native speaker of US English. Due to the nature of the post (they knew their elderly relative wouldn't be with them much longer because he was passing out the Benjamins like sweets) it doesn't feel appropriate to ask the poster.
I've tried googling it, but that just gets more examples (and always with the upper-case B) rather than any explanation.
Can anybody give me a clue what it means? Is there literally a thing called a Benjamin that people give to other people? Is it some kind of metaphorical usage? And is it modern, old-fashioned, regional, etc?
ETA: I've now been informed that "Benjamins" are hundred-dollar bills, due to there being a picture of Benjamin Franklin on them. Thanks for the very quick response! But I'm still interested to know whether this is a modern, young-person's usage, or whether it's standard amongst all age groups; regional or national; slang or acceptable in formal situations, etc, because this is not a phrase I've vaguely wondered about for years, this is a phrase I have literally never seen/heard until this morning.