Hi Ivan,
This is just an observation.
When you have sold the material to customer, that mean you have billed him. Even though you are keeping the stock at your place, You should have made the stock movement also. So that, your stock of materials reduces and all proper accounting entries happens of the sale, be it COGS, Inventory account, stocks (quantity. ).
Now the other issue comes of storage. Here you have to have a storage location probably outside sap to keep this goods. As this stock is not gonna be affecting any of your accounts or processes, why not keep it outside. And if you are doing this as an off case, very temporarily, tha'ts the best option.
If you want you can have a storage location for that but the stocks in it should not hit any of your accounts. MM guys would give a solution for that.
Thanks
Jobi