Hello
BAdIs and exits are usualy called before the database update, as they are generally used to update or check data that will be posted.
Table RESB is saved on form RESB_POST of program LCOVBF10 and you can check if there is any place to your enhancement here.
Be very careful, as a modification here may lead to update terminations or inconsistent data saved to the database.
BR
Caetano