Entity reference, save only value I am looking for the best approach to do the following:

I have a content type Product and a content type Invoice, where i can reference products.

I would like to be able to save the referenced products AT THAT TIME (with their price info).
So when the product is changed in the future, the invoice will not be updated with it.

Any suggestions on how to approach this best?

Thanks in advance!
as far as i can see there are 2 options, and both would involve writing some custom code. 

1st, provided that you have enabled product revisions, when saving the invoice, save that particular revision number and then load your product based on that revision. 

2nd, would be to add product fields that you are interested in, to you invoice (price, desc etc.) and based on the product attached save those in form validation handler