Is my testimony considered false and will I be sinful, and what should I do to atone for myself after I testified to the sale of a piece of land for eighty thousand, knowing that the seller said he gave his sisters his land and took their land and they forgave him the difference in price?

The tongue must be protected from bearing false witness, which is to testify to something you do not know, or to something you know to be the opposite, even if no apparent harm results from it. Whoever does this is obligated to repent to Allah Almighty and to rectify what they have corrupted by their signature if corruption resulted from it.