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Could it perhaps be "Run Lola Run"?
Edited by RedakaA bad guess, but this made me think of the airport shooting scene in 12 Monkeys.
If you excuse a little muddling of the details, it could possibly be Looper. Did the slow-mo bullet scene happen in a cornfield? There's a fair amount of arguing over money, although none of it is in a suitcase that I remember.
My mum keeps talking in the vaguest details about a film she barely remembers seeing. There are only ever two details she mentions:
1. A homeless man (presumably the protagonist) who by accident gets his hands on a suitcase full of money and spends the film trying to get away with it.
2. A pivotal scene involving a bullet being fired in slow motion. Now, this could be any number of films with bullet time, but my mum puts a lot of emphasis on that scene, like it's the centerpiece of the film or something. Apparently, depending on who that bullet hits, the film would have a very different outcome.
I know these aren't much to go on, but the "bullet film" has become a massive running gag in our family, and we all - including my mum - would love to finally figure out what in the world that film is.