What is the easiest way to pull a custom field from the child record (item receipt) onto the parent record (purchase order)?

It's one field on the purchase order? What happens when there's more than one item receipt? What value should go into that one field on the purchase order?

Good point... generally we have only one item receipt but there are exceptions. are you saying it is not possible to pull this information then?

Just because you have a business rule where there's just one item receipt, the fact that NS allows multiple means that you can't just source in one value. Unless you did it via script where the logic would be "find the first item receipt and grab that value".