I have a workaround installed now. It's not pretty though.
Created a second temperature sensor in my sketch and did the mathmetics there also. In Vera I monitor this device for +2 or -2.
send(msg.setSensor(0).set(temperature,1)); // Custom temp_diff=temperature-lastTemperature; Serial.println("Temperatures compared: "); Serial.println(temp_diff); send(msg.setSensor(1).set(temp_diff,1));