While we can create a Receipt document using the WebAPI, there is no way to have the receipts associated with an existing (or even a new) purchase order number. Why would any business receive in goods without an associated PO? They wouldn't. By fixing this issue, you allow Sage 300 to integrate with many 3rd party shipping services and warehouse systems. This is clearly a bug - and was discussed further on Sage City:

https://www.sagecity.com/support_communities/sage300_erp/f/sage-300-general-discussion/110311/error-when-trying-to-post-a-multiple-po-receipt-using-sage-web-api

by: Paul T. | over a year ago | SDK