@alasdair is entirely correct. Sometimes a company keeps stock in its own warehouse, even though a customer has paid for it. (Perhaps the company acts as a fulfillment center for the customer.) But the inventory legally belongs to that customer, not the company. So the company cannot transfer it, because it isn’t on the company’s books any longer. The exact moment when ownership passes depends on the terms of the contract or purchase order. It might be when you issue the sales invoice, or it could be when they pay the invoice. Legal advice on this issue could be valuable.
Such retained inventory situations are legally tricky. Who is liable for loss or damage in the warehouse? Who insures the inventory? And why would you transfer something one customer bought to another customer? Hopefully, you do that only at the first customer’s direction, because it is really their sale to make, not yours.
Courtrooms and prisons are full of managers who have illegally juggled inventory to make it appear like they have met sales quotas, achieved production milestones, and so forth, often to favorably influence stock prices. That constitutes fraud at minimum and possibly larceny and other crimes.
It’s probably best that such transfers are somewhat inconvenient. They should not be made in very many situations at all.