There is an expression in (British) English along the lines of
I have to go and see a man about a dog.
It is the thing you might say, for example, when leaving a gathering of friends at the pub and you want to forestall any enquiry about what you will be doing, or why you have to leave. It is different from saying, "Mind your own business!" specifically because it can be used pre-emptively. It is also much more friendly.
Is there a similar idiomatic approach in German? ... and in addition, how does one tell someone to mind their own business!
N.B. Thanks to @RDBury in the comments below for the link to the Wikipedia explanation!