Is testifying to a sales contract, without having been present at the agreement or the handover of the money, considered a false testimony, and what is its expiation if it is so?

If the seller and the buyer acknowledge the sale in your presence, and you bear witness to that, then your testimony is valid. You are not required to know the details of the contract or to be present during the delivery of the two عوضين (countervalues/considerations); because the intent is to bear witness to the occurrence of the sale, and it has occurred.