Hi naomiacute
A Promised Date can be entered for an invoice by opening the invoice and using that Promised Date field listed on the left-hand side of the Edit Sales window. When a customer is on credit hold this locks the ability for any new transactions to be added and any existing transactions to be edited. As a result, you would need to remove that credit hold and then re-add that back in, if necessary.
To my knowledge, we don't have any immediate plans to change that functionality to allow the promise date to be added to already existing invoices when the customer is on credit hold. This may be something that we see in the future but not currently within the scope of the product.
