set condition when end user selects 'BACK' button in CJ20N transaction

hello all,

we have a Z-transaction (from which the end user can create a new WBS number or edit an existing WBS number by calling CJ20N ) ,

after calling Cj20N as shown above , when the end user clicks 'BACK' or 'CANCEL' or 'EXIT' the control is going to the Z-transaction ,

but my condition is when 'BACK' or 'CANCEL' or 'EXIT' is selected instead of directly going to Z-transaction , i wanted to check for some condition

i.e if there is no network number then it should not allow going back(when user selects 'BACK' button)...

how do i achieve this condition...

thanks in advance...