Does the absence of witnesses in a sales contract invalidate the contract?

The absence of witnesses in a sales contract does not invalidate it. Rather, the contract remains valid and its effects take hold if it fulfills the conditions of validity and is free from فساد (elements that corrupt it). This is based on the established fact that the Prophet, peace and blessings be upon him, bought a horse without witnesses, as indicated by the hadith of Umarah ibn Khuzaymah. As for the command to witness in the verse, "And take witnesses when you trade," it is not for obligation but for guidance, by the consensus of the jurists.