User Exit???

hello Experts,

I wanted to knw a user exit for T.code vf01 which can be used for number range generation .This has to trigger after the Billing document gets generated i.e it is saved??...Plz do the needfull as early as possible.I hav used RV60AFZZ include pgm which contains a user exit for number range ...but i need to generate no. after the billing docu is saved ...but this gets triggered before the billing document is generated...plz reply as early as possible.

Thanks,

Reshma