Multiple quantity of item- Manually creating serial numbers during RAF + Automatic assignment during picking/packing slip

Hi Guys, 

I have a requirement to assign serial numbers to the finished goods during report as finished (RAF) but the client wants to maintain its own serial numbers which are not in sequence (they maintain some random numbers depending on the production line etc. ). After that these numbers should be assigned automatically during sales process. It works fine if the quantity is one, how to handle this case in case of multiple quantity scenario: 

In other words: 

Production

1. Maintaining serial numbers in the system for 10 quantity 

2. Assigning the numbers during RAF ( I am not able to find this one) 

Sales Orders

1. Assigning serial numbers automatically during picking/packing. 

Hoping for some solution from the experts.