badi WORKORDER_UPDATE

Hi,

i´m using badi

WORKORDER_UPDATE

.
i´m trying to check field AFVGD-USR00 (operation, details, field materials group) before saving the changes to this field.
i´ve essayed to do it in method

BEFORE_UPDATE

but the saving takes place before because i get the corresponding message of the changes being saved.

Any idea of how can i solve this?

that is, where do i have to implement my code in order to check the field before changes being saved?

Best regards.