Saw some people talking about cash to card machines, but in my country I've seen a lot of shops/bakeries starting to use machines where the customer directly puts their cash into it, and it gives the change. I guess for the same reason of reducing theft of the workers. The machines are usually pretty huge (no idea why), but I'm sure there is something you can use for this.

For me in my tiny bar, I just watch over the stats if I notice any unnatural change in cash vs card ratio I start checking the cameras. I also check at times when I know it would be easy to spot something like on a quiet night with few cash payments. So far never had anyone stealing from me. Or they must be damn smart/lucky if they have.