Kesubos 82a - Kinyanim: Meshicha, Kesef and Shtar

The gemara concludes that if one makes a kinyan now so that the act of the kinyan will be done now to be effective in 30 days, and the chalos of the kinyan goes into effect retroactively from now, it is effective. However, if the chalos of the kinyan was only designed to go into effect at the end of the 30 days, Tosafos explains that it would depend on the type of kinyan. Meshicah which is an act of kinyan and has not lingering effect, the chalos and ma'aseh kinyan must be at the same moment. The Ran disagrees with this conclusion (see ketzos 127:3) by maintaining that the gemara is merely developing the opinion of R' Yochanan who limits the statement of th R' Dimi to where one said "mei'achsav", but other amoraim argue and hold that one need not say "mei'achshav".

Nevertheless, even Tosafos agrees that when it comes to kinyan kesef, it can be done today and only go into effect in 30 days without stating and retroactive chalos. The distinction between kesef and meshicha is that the kinyan of meshicha is accomplished by the act of meshicha, and once that is over there is no lingering effect. However, the very nature of a kinyan kesef is not the act of receiving the money, but rather a willingness to give up an item in exchange for the benefit received from that money. Therefore, so long as the money will have to be returned if the kinyan is not achieved, it is as if the money was actually given at the end of the 30 days (the question still remains how this works by kiddushin where we hold that mekadesh b'milva eina mikudeshes; would this be like mekadesh b'hana'as mechilas milva?).
Tosafos also indicates that a contract would be similar to money, that if it is still around at the end of the 30 days the kinyan is effective even without a clause of mei'achshav. This indicates that a kinyan shtar is not achieved by the act of handing over the contract from seller to buyer (as by meshicah), but is rather achieved by the buyer being in possession of a contract that states the land belongs to him. Therefore, so long as the buyer possesses such a contract at the end of the 30 days, the kinyan will go into effect at that time.