Does buying after the Farewell Tawaf—due to an urgent need, thinking it won't be available later, and in ignorance of the prohibition against buying except for necessities—constitute an error that necessitates anything?

The pilgrim performing Hajj or Umrah (definition) should make the Farewell Circumambulation (Tawaf (definition) al-Wada') his last act in Mecca, and depart as a traveler immediately thereafter, based on the Prophet's (peace be upon him) saying: "Let no one depart until his last act is circumambulating the House." There is no harm for someone who buys something on his way out after the Tawaf, as long as he does not intend to stay, such as buying travel necessities, a gift, or a book. However, if he engages in trade, he must repeat the Tawaf.