serial number validation during return delivery

Hi All

I have one requirement in which i assigned serial numbers to items during delivery

than invoice generated after delivery than customer return that items so we create return sales order then return delivery so during return delivery i need to validate the serial no. enter during first time and the same serial no. must return0 during return delivery ...so where l'll validate this

i hav checked this program MV50AFZ1 and checked all exits in this program but i am able to get serial no. during first delivery but now during return delivery i m not able to get the return serial no. values since delivery has yet not been created..

so how can i do that

pls guide

Thanks,

Taran