Hello MANI,
Suddenly I recollected right user-exit and the right solution. The user-exit is IMRC0004. Put the following code in the include ZXMRCU04 .
IF sy-tcode = 'IK11' AND impt_data-mptyp = 'Z' AND impt_data-indct = 'X' AND imrg_ins-idiff IS NOT INITIAL.
MESSAGE: 'Reading in ''Difference'' field is not allowed. Please enter your reading in the ''Counter reading'' field.' TYPE 'E'.
ENDIF.
This throws an error in the status bar the moment someone enters a reading in the Difference field for 'Z' category of measuring points and hits Enter.
Besides very simple coding, there are no hassles of Saving, or Dumps as in the case of previous exit IMRC0001. I'm sorry for forgetting this solution at the time of answering previously. I'm sure, this solution would suit to you.
Good luck
KJogeswaraRao