If you forcefully set the Quantity (to deliver) in the outgoing Transfer, Odoo will let you. It wouldn't make sense otherwise - since, for example in cases of incorrect stock levels in Odoo, when staff 'just finds the product lying around somewhere', you wouldn't be able to deliver it, regardless of literally holding it in your hands.
Odoo tells you in the outgoing Transfer however, that, based on the stock levels it knows, nothing can be reserved, thus the product is Not Available:
However, if you, as stated above, forcefully set Quantity, Odoo will accept it as is:
This has nothing to do with Manufacturing, but simply with not blocking you, when you are sure you can perform a transfer.
Your On Hand Quantity, based on what Odoo knows about at that point, has to be reduced - in this example of a new (manufactured) Product, the available stock becomes negative:
Therefore, when manufacturing is completed, your Quantity On Hand will be set to 0 - 1 sold and shipped, 1 manufactured = 0. Future cases are therefore not affected.
Only thing you could do at this point would be to Return the outgoing Transfer from your customer, so your stock level temporarily shows +1 just to intimately after perform another outgoing Transfer to your customer.
Still, the history has been written to the database on the 14th, so all you would be doing is creating additional moves from b to a and a to b, further obfuscating what happened for any future viewing of that Sale Order.
You could be doing, on the other hand, a manual inventory adjustment, yet, this doesn't make sense in your case since, based on the records, the Good was just not manufactured at the time of delivery.
All that being said, if there is no actual reason of why to dig deeper at that point, just leave it be and put a Note in the Chatter stating what has happened - and ensure you're not forcefully setting Quantities on outgoing Transfers future onward.
Note: When using Serial Number-based Tracking of your Products, performing such wonky Transfers could be reduced (by specifying that a new Serial Number can be generated in Manufacturing Transfers only, and Outgoing can only use existing ones - aka, on stock):
Also, this might be interesting to you: https://www.odoo.com/forum/1/272531
