Rabbi Meir said

any condition that is not like the condition

of the children of Reuven and the children of Gad

is not a [valid] condition

as it says:

"And Moshe said to them

'if the children of Reuven and the children of Gad cross over [the Jordan with you zealously . . . then you shall give them the land of Gilad] . . . .

and if they do not cross over with you zealously'" [they would not get the land of Gilad].

Rabbi Channaniah ben Gamliel says

it was necessary for it to be said [in that fashion]

as otherwise

it would imply

that even in the Land of Cannan they would not inherit.

One who betroths a woman and says

"I thought that she was a Kohenes

and she turned out to be a Levi"

or a Levi and she turned out to be a Kohenes

or a pauper and she turned out to be rich

or a rich woman and she turned out to be a pauper

she is betrothed

because she did not mislead him.

One who says to a woman

"You are hereby betrothed to me

after I will be converted"

or "after you will be converted"

or [if he is a canaani slave] "after I become freed"

or [if she is an canaani maidservant] "after you become freed"

or "after your husband shall die"

or "after your sister (who is his wife) shall die"

or "after your yavam shall do chalitzah"

she is not betrothed.

And similarly one who says to his colleague

"If your wife gives birth to a girl

she is betrothed to me,"

she is not betrothed;

but if his colleagues wife was pregnant

and her fetus was discernable

his words have effect

and if she gives birth to a girl

she is betrothed.