At the moment, your options are limited, in that Airtable does not currently support persistent calculations (that is, a formula field that, once calculated, retains the value even if the value of variables used in the formula change — essentially the equivalent of an Excel cell with automatic recalculation toggled off), so you won’t be able to lock the ID. It also does not presently support enforced unique values for a field, so you can’t guarantee non-duplicate items.

That said, there are workarounds that may or may not prove workable, depending on your need and application. Typically, if I want to force persistence on a calculated value, I’ll mirror the formula field with a compatible data field and physically duplicate the value from the impermanent formula field to the more] permanent data field.

If the fields are adjacent in a grid view, such duplication can be performed with a three-stroke Ctrl-C/RtArrow/Ctrl-V sequence or by selecting and dragging the fill handles one cell to the right. More importantly, a Zapier Zap, Integromat Scenario, or the equivalent can be defined to duplicate the value automatically whenever a new record is added to the base. Presumably, a more-complex Zap or Scenario could be created that could receive its inputs from Airtable, mull them over, and either enter the appropriate ID or generate a new one, as needed.

For sake of your users’ sanity, I suspect using a third-party SaaS integration service would be your best approach – either that, or come up with a different ID algorithm. That’s a reasonably sophisticated process flow your current accounting package performs, and it would be very difficult, if not impossible, to replicate it in Airtable as the platform now stands.