Hi
Instead of going for ABAP check why don't you control this with authorization access?
In most of the companies storage location for cash sales is different from others. If this is same in your case you can control this by not giving authorization of 602 movement type to the users for that store.
If possible movement type for cash sales could be different.
Shipping point could be different.
Check it with this perspective and see if anything can be done in standard to control this.
Thank$