I've never heard that expression. The only thing that comes to my mind, is that they're offering significant discounts to people who pay with cash. The idea would be that if you pay with cash they will "slash" -- that is cut significantly -- the prices.

are you sure you don't mean "stash in cash", which does mean a lot of cash?

As far as I know, neither 'slash with cash' nor 'slash in cash' means anything at all. I googled the words, and all I found was a photo of a rock musician called Slash with a child called Cash. In addition to that, there's apparently a video game called 'Slash for Cash', where you have to cut up sushi to earn money.

The correct term is "flush with cash".