"I bought this thermos for free" Seems properly regular to me. "That thermos was for free" Appears undesirable: I'd personally just say "That thermos was free." The phrase is normally inaccurate. If You will need to purchase just one to have the future 1 for free, it was not essentially https://docs.google.com/document/d/e/2PACX-1vTrVZBlXQ93932G_bwrC4lvRYQ9hi-BzXccPrzn0BlNgJSiUFDowMTDkoAEyzcR1K8T6eyq9rdwdvgC/pub