Is it possible for a workflow to set a transaction body field value on an invoice based on the value in a custom field on the item record from the first line item on the invoice? In other words, if I have an invoice for item ABC, is it possible for a workflow to reference a custom date field on the item record for item ABC and use that value in a formula that will calculate the due date of the invoice?

Yes this should be possible a few different ways to accomplish this. I would recommend a date field with a sourced value being the date from your item record Then use that to set your due date

@AI1 Do you mean that I should create a custom transaction body field on the invoice record that references the date on the item record, and use that custom field in a workflow formula to set the due date on the invoice?

Yes that is correct

@AI1 What if the invoice has multiple line items on it? How would I tell a workflow which item's date field I want to use?

The custom field on your invoice should have its value sourced from a saved search. You would just have to create a saved search that has the criteria to get you to the item you want. I think earlier you mentioned just wanting the item from the first line on the Invoice Maybe make a saved search where Line is less than or equal to 3 to filter down to just the item on line 1