Follow-up with new information that came in on https://community.blackbaud.com/forums/viewtopic/494/60432 ; when creating gift records through a batch, it is also the case that the "lookup_id" passed in is not retained during the creation of a batched gift, or applied when approving the batch record.

This means that it is also not possible to set a lookup_id on a gift, and then refer to that created gift in Raiser's Edge NXT after the batch has been approved.

There might be something you could do with custom field categories, by commandeering a custom field category to act like a lookup id would, but doing so adds extra complexity and requires some "Environmental Admin" endpoints in the NXT Data Integration endpoint section.

Adding in more details for a specific use case (also logged in https://community.blackbaud.com/forums/viewtopic/494/60432 )

It appears, when making a new batch, and adding a gift to that batch, the pending gift that gets created for that batch has its own id. This id is returned when you add a gift to a batch.

However, when you go to approve a batch, the "approved" gift no longer has the same id it had when it was in the batched state.

As such, it is not possible to connect the "create batch gift" with an approved gift, using the id returned when adding it to a batch, because the ids are different.

I'm not sure if you can access the unapproved batch gifts at any point, but when you first create the gift using Gift (Batch) endpoint, you get the gift's id in the response. I've used it to successfully add custom fields to unapproved gifts.