In the Transfer section of the Inventory App, my customer wants to have all serial numbers of same product concatenated in the same field in their delivery slip. 

See the way to do that: 

1) Do your transfer with products and serial numbers. The serial numbers will be allocated in the "Detailed Operations" tab. 

2) You have a "Serial Numbers" many2many field hidden in the "Operations" tab. Those SN's are taken from the "Detailed Operations" tab and are concatenated. 

3) Go to the report "Delivery slip" in Studio

4) Retrieve the "Serial Numbers" field. Small trick here as the "Serial Number" does not seem to be available at first glance (Bug ?)--> choose a Field column and insert it, and choose a random path stock Move --> "whatever field that has a data" (example: Forecast Availability). Then, from the left panel, you can select another path which somehow has more fields available. You then replace the path Stock Move --> Forecast Availability by Stock move --> Serial Numbers (and replace the title). Then the magic happens. 
Hope this helps and explanation is clear with below screenshots. 

Cheers,