Hi,
As far as I know you cannot work with a "normal" variable together with an offset of -2 periods. It is quite usual that Fiscal Year Variants have one or more special periods (e.g. 13, 14, etc.). The calculation has to take those special periods into account.
I suggest to create a customer-exit variable as select option, not input-ready so called after the variable pop-up (i_step = 2). You can use the Function Modules DATE_TO_PERIOD_CONVERT and RST_TOBJ_SHIFT in your coding.
Best regards,
Sander