Partha,
You haven't given any infiormation on how the user interface fields are implemented so it will be difficult for anybody to give you a definitive answer. Is the code using SCREEN SECTION, Dialog System, Windows Forms or something else ?
If it is SCREEN SECTION you could set the TAB key to terminate the accept, using the key mappings in ADISCF, use the CURSOR IS clause to determine which field you were on, do the required validation and then redo the ACCEPT/DISPLAY with the CURSOR positioned at the rquired field by updating the CURSOR IS value.
Gael