It is not possible in Unleashed to create a Supplier Return without the original Purchase Order existing (and being completed) in Unleashed too.

Each Supplier Return in Unleashed is created using the original Purchase Order as a template, to initially include all products, stock, prices and costs. The Supplier Return can then be edited and adjusted to align with the return's requirements before completing. This ensures the credit being supplied for the purchase is accurate and accounted for correctly. 

For more details on managing Supplier Returns in Unleashed, see Add Supplier Return.