An important halachic paradigm appears at the end of parshas Mattos this week. As our ancestors sit on the edge of Eretz Yisrael, ready to make their way across the Yarden river, the leaders of the tribes of Reuven and Gad approach Moshe Rabeinu, a"h, with what initially seems an outrageous request. Given their large number of livestock and the suitability of the local land for it, they want to stay put. Everyone else can go across the Israel, they'll stay on the East Bank and do just fine.
What follows is an extended dialogue with Moshe Rabeinu demanding that they abandon their plans for fear of demoralizing the rest of the people and the two tribes replying that they would function as the vanguard of the invasion force, staying to help their brethren conquer the land before returning to settle in their current location.
Quite an extended dialogue and in fact, the Gemara learn from this conversation in which the same thing seems to get repeated over and over about the important rules of giving something to someone else al tanai, on condition. As Chazal state and as is brought down in Shulchan Aruch Even HaEzer 38:2, if you are giving a conditional gift to someone you must do it in the form that this deal was worked out in.
In short, there are four conditions that must be fulfilled, as derived from the narrative:
1) It must be a double statement: "if, then" and "if not, then" detailing what happens if the condition is or is not fulfilled
2) The positive condition must precede the negative
3) The condition must precede the consequential action in the wording, not the other way around
4) It must be a condition that is possible to accomplish
In the absence of any one of these stipulation the tanai is null and void.
So the question might be asked: if one gives a gift to his fellow he said the following: "I'm giving you this if you do such-and-such" and the recipient agrees to perform the action and promises to but then doesn't, can the giver demand his gift back even though he didn't fulfill all four stipulations?
At first glance the answer, following Even HaEzer above, would be no. However, there is another relevant spot in Choshen Mishpat 241:9 that brings a different impression:
"Anyone gift given on condition, whether its the donor's condition or the recipients, and the recipient acquired the object, if the condition is fulfilled the gift remains with the recipient and if not, the gift goes back."
In this case there is no detail as to how the condition should be made. One possible explanation is that Choshen Mishpat is relying on the details given in Even HaEzer. Another is that the two parties didn't know about the law in Even HaEzer and were making arrangements as they understood it. In such a case the rule of returning the item in the event of lack of fulfillment of the condition applies even in the absence of the criteria from Even HaEzer being satisfied. This is because both parties agreed, as it were, to forgo them.
The Rambam and the Rosh hold that these four criteria apply both to cases of issur v'heter like kiddushin and to financial transactions but the Tur disagrees with them and differentiates between land and movables. In a similar vein, the Netivot HaMishpat brings a similar answer from the Ateres Zvi that the four conditions apply only to gifts of land, not movables.
But if Chazal said that these four conditions are required to enact a truly conditional transaction, why are there so many disagreements on how and whether or not to enforce them? Some authorities note that demanding all four criteria from everybody all the time is quite burdensome. The frustration that such enforcement would cause, especially because most people aren't aware of these rules, would lead to widespread abandonment of any conditional rules. The Tashbetz therefore bring that we don't demand fulfillment of all four rules in financial transactions but we still do when it comes to issur v'heter.