Does Google Fi match serial numbers to users on new phones (for promo purposes)?

My sibling and I are on the same plan. Recently we each ordered a new phone and we both got a promo for ordering the said phones. At the check out screen Fi asked us to match the phones to our lines for promo to apply. The phones are the same model but different colors.

The phones got here and they didn't like the color in person, so they plan on returning. I wasn't super impressed with the phone and wants to return to my old phone. However, they liked my new phone's color. When they tried to order my color the promo isn't there, since they already used it to buy their new phone (soon to be returned).

So my question is, can I just give them my phone? Will we be able to keep the promo price even though it won't be activated under my line and instead is being used on theirs?