The way to do this is to define two stubs: 1) with the specific URL, method and request body matchers for the positive response, 2) lower priority (> 5) with no body matchers for the fallback response.
The 1st, higher priority stub will be matched if URL, method and body are correct and you'll get that response. Otherwise it'll fall through to the 2nd stub and you'll get the response from that.