Rabbi Meir says in the Mishna on Kidushin 61A that the rules of formulation of conditions in a contract which are derived from the formulations of the conditions that Moshe Rabbeinu stipulated in the case of the Reuvenites and the Gaddites are binding, and if not followed exactly - the conditiones are not valid and the contract is valid.
How does Rabbi Meir know that Other ways are INvalid? All we can know from the method used by Moshe Rabbeinu is that this way IS valid - but that des NOT prove that other ways are invalid!
Moshe Rabbeinu could not have used more than one method - so his choice of method is no proof!
If Moshe Rabbeinu would have been reported to make various contracts and always used this type of formulation - perhaps the inference would be indicated, but as the only source - how can this be considered a proof of the invalidity of other formulations?
See Tosfos, who explains that had the other way been acceptable, Moshe should have said the other way. As far as Kaful goes, it is obviously unnecessary except where it is needed. But Tosfos seems to say that Ma'aseh Kodem l'Tenai would be the obvious choice as well as Lav Kodem l'Hen, had they been acceptable.
D. Zupnik