who told the foreman it was OK?. If that person is higher in the company than those requiring reimbursement for it, have them tell the folks to back off. If the requesting person is of greater position, apparently the person who told the foreman it was ok did not have the authority either.
precedent does not make any difference. Just because I give you my old shoes a time or two does not mean you are always entitled to them. They apparently changed their mind.
If the copper is old demo stuff, the contactor sometimes figures that into the bid. If it is new scrap trimmings, that IS company property and whomever directs the company has every right to demand all scrap be returned, after all, they did buy the stuff.
So, although it does not seem you intended to steal, apparently the rightful owner wants the copper so you have to give it up.
as far as grand theft; how much copper are you talking about? grand theft usually has a threshold of value to meet before it is applicable.
I have also seen electricians intentionally scrapping good wire simply so they could sell it as scrap. Maybe somebody on your job did this or management beoeived it to be so and instituted the rule they have now.

