Q: Is a cat toy with catnip in it OK to let a ferret play with?
A: Because catnip does not affect ferrets like it does most cats, playing with catnip should be OK. I would be more concerned with the toy itself and not the catnip.
Make sure the toy does not have parts that your ferret could break off and ingest. If there are rubberlike parts to the toy, a cat might not try to eat those but a ferret would.
If possible, always purchase toys specifically designed for ferrets. It is much safer for the ferret and it will give you peace of mind when your ferret is playing with a toy and you are not around to watch.