Do not update a lookup

How can I make a value entered through a “lookup” not be updated ?
That is, if the source value of the lookup changes, do not change the value of the linked field.
Thank you for your help and patience with a beginner like me.

You really can’t – a lookup field always and only reflects the value of the field you are telling it to lookup.

The only real work-around I can think if is to make another static text field next to the lookup field and copy-paste the value from the lookup into the static text field when it makes its initial lookup (it’s initial state). Then, while the lookup will change when it’s referenced field changes, your static text field next to it will not change.

an example for what I want to do.

I have a table of products and each product has its description and price.
I also have a billing program.
When I make an invoice, I select the reference of the items I want to include in the invoice and through a lookup field, it gives me the price and description.

Ok, now let’s suppose that I am going to change the prices of some products.
After doing so, all the old invoices will update the value of those items. That changes the amount of invoices that are already collected and sent.

How can I avoid that when I change the prices of some items, I update the old invoices?