Torah Temimah cites Yerushalmi Sanhedrin 3:9.
A single witness to whom the parties said “you are accepted by us as two” is not within the scope of shevu’as ha-eidus, for it is written “and he is a witness” — meaning Torah testimony (two witnesses).
The Yerushalmi excludes party-accepted single-witness arrangements from the liability structure of the verse. Only testimony that meets the Torah’s two-witness standard triggers the oath-and-korban consequences of Vayikra 5:1.