Keep the calculation field from continuously updating field

Title

Keep the calculation field from continuously updating field

Post

Hi there,

Using fmpro12 for invoicing.  using calculation field to lookup price of product.  Is there a way to prevention the calculation field from updating a paid invoice?  ie.  Allowing the calculation running only once?

I tried wrapping the formula if(price="", price formula).  Didn't work.

I searched the forum.  I found article mentioning about changing user rights to restrict access to the invoice record.  Not really what I am looking for.  I need to prevent calculation field on paid invoice from updating when there is a price change in the future.  Not worry about user.  I am worry about the calculation field.  any ideas?

Thanks in advance.

Henry