The answer: he takes it with a new term, because one of the characteristics of the price is that it be deferred. Thus, he takes it with its term beginning anew. So, if it is assumed that he sold it three months after purchasing it, while the term was six months, he begins a fresh six-month term, because that is the price.
His statement, “or from someone whose testimony is not accepted in his favor,” means: or if the seller purchased it at a disclosed-cost price from someone whose testimony is not accepted in his favor, then sold it to another without informing him, the second buyer has the option. This is because a person will generally not scrutinize the price closely when dealing with someone whose testimony is not accepted in his favor; ordinarily, he will not bargain hard.
Those whose testimony is not accepted in his favor are his ascendants and descendants: fathers, mothers, grandfathers, and grandmothers; sons, daughters, sons of sons, and sons of daughters. Likewise, the testimony of either spouse is not accepted in favor of the other.
For example, a man purchases an item from his father for one hundred, then sells it to another at its cost price. The buyer subsequently discovers that the seller had bought it from his father. The author says: he has the option.
The apparent meaning of his words is that he has the option whether or not he was overcharged, based on the fact that the customary practice is that a person does not scrutinize the price when purchasing from someone whose testimony is not accepted in his favor.
The reason the buyer is given the option is that, had the seller purchased from an unrelated person, he would have haggled and bargained him down. With his father, however, he would be too embarrassed to do so; the same applies with a son.
The correct view in this issue is that the option is not established for him unless it becomes clear that there was overcharging. In that case, he has the option, and it falls under the option due to overcharging. But if there was no overcharging, then people often purchase from their ascendants or descendants and still scrutinize the price carefully.