Rashi says it's not Shtar Mukdam but in Divrei Hamaschil "Nikneh Hashtar" he says the Shtar is nikneh with the Chazoko . Surely the kinyan is at same time as Chazoko and therefore the Shtar is Mukdam ?
Moishe Weissbraun, London England
The Rashi you quoted gives two possibilities why this Shtar does not have the Halachah of a Shtar Mukdam (even though it was clearly written chronologically earlier than the actual deal). One is that the seller actually enacts a Kinyan for the buyer on the Shtar (not the field) when the Shtar is written (many Rishonim argue that this does not seem to fit with our Gemara that states he acquires the Shtar later, see Ramban). The second is that as long as the witnesses sign the Shtar later, there is no problem of writing it earlier (as per the opinion of Abaye in Bava Metzia 13a).
All the best,