I have never heard anything about 6 toed cats - but in Japan there is a lot of folklore related to cats.
Here, it`s said that if you do a favor for a cat, in time the cat will pay you back. Recently this has come to be interpreted as normal cat behaviour - leaving mice as gifts, etc... But in the past it had much stronger overtones of magic and the supernatural. As cats were said to be able to transform into humans, kindness to a cat could possibly lead to some significant events in your favor that appear to have nothing to do with the cat itself.
On the other hand, it`s not always true that what a cat would consider "good" repayment would be considered the same to a human... And folklore abounds with examples of the mismatch in these worlds.