This is a complete code example of how to code a Purchase Receipt in Web Services
The main call here is this line:
' Create the purchase invoice
Try
wsDynamicsGP.CreatePurchaseInvoice(purchaseInvoice, context, purchaseReceiptCreatePolicy)
Catch ex As System.ServiceModel.FaultException
Throw ex
Catch ex As Exception
Throw ex
End Try
This is a complete class, just create a class in Visual Studio called CreatePurchaseReceipt and drop it in. You'll need to edit the PO number, item number, etc.