If you only want the first byte then no need for the loop. Change
for(byte i = 0; i<len; i){
// sprintf(msgString, " %.2X", rxBuf[i]);
T=(rxBuf[i]-40);
Serial.println(T);
delay(1000);
}
to just print out the first byte.
T=(rxBuf[0]-40);
Serial.println(T);
delay(1000);