Drop ship advice / help

Question

Hello, hopefully someone can help!

The company I work for have distribution centres in a few regions globally, and we also have some legal entities (sales offices) in areas where we don’t have a distribution centre.

One of our APAC distribution centres in Singapore, will supply goods to customers in Hong Kong (we also have a legal entity in HK). The customer will order the goods via the Singapore business unit and the Singapore DC will ship the goods out with the correct paperwork etc with the HK entity being the IOR.

If Singapore does not have the stock , then they will replenish it from our UK distribution centre, before then being processed in Singapore and shipped to the end customer (time implications in doing this). The UK sells the goods to SG for this transaction and is reflected in the import / export paperwork.

What we want to do during a stock out in Singapore is to switch the order to the UK (to cut out the replenishment) and direct / drop ship to the end customer, however the transaction still needs to remain as the HK customer buying from SG, and SG buying from the UK. What paperwork would we need to send this to the end customer (assume commercial invoice), and what costs would we use (HK buying price from SG, or UK selling price to SG) ?

Hope that makes sense!!

Thanks in advance