It’s difficult to say without getting more information and ideally some sort of visual example of what your current setup is.

List A is a list of Requests, and those Requests seem to go through some sort of Approval Process and are given a ‘number’ as part of this process. Is the Approval Number unique or is it more like a status picked from a choice field? Do you use a Nintex Form to input this Number or is it automated by way of some other process or workflow?

List B is a list of Invoices. Are there Multiple Invoices that are related back to just 1 Request by way of its Approval Number, or is it a One to One (One Request to One Invoice) relationship? Are the Invoices created after a Request has been approved or are Requests and Invoices created at the same time and manually related later?

It’s difficult to know exactly what you want or where you want the work to be done (on a form (Classic, Responsive, or NEW Responsive?) or performed by a workflow to show something in the list) without just a bit more detail.

In list B, have a workflow that sets a field in List A called “Used” when the approval number in A is used.

Create a list view of A called “Open” and filter out any items where B has set the Used field flag in A.

The form for B would not have a freeform field for the approval item from A, but instead use a list lookup form control back to list A, using the filtered view “Open”.  That way a user in B could not select an item in A that has been used…

Also, in the SharePoint list settings for B, set the approval info field in B to Unique values...