how to check investment program when use CJ01 create a project before save?

dear all,

now i want to check investment program when use cj01 create a project before save it.

now i create a project use cj01 and a wbs,when i assign a investment program for the wbs.some user can't use scale 91.so i want to check the investment program before save it.if it have a wbs's investment program scale is 91,alert users they can't use this investment program.i do a customer exit can do it when user click save buttton,but when user click back button,pop window show.prompt user save it.if user click yes,the project can save as normal.

how to prompt user can't use investment program scale 91 when user click back and choose 'yes' on the prompt window?

thanks

best regards.

urgent.