One who sets aside produce

to separate with them

teruma or maaser [as a substitute for the teruma or maaser he would have to separate from crops that he has elsewhere],

[or if one set aside] money

to separate with them

maaser sheini,

he may separate with them [and consume the crops]

on the presumption that [the substitute crops or money] still exist.

If they [the substitute crops or money] are lost

he must be take into consideration from one time to another (i.e., he must assume that the substitute crops or money were lost 24 hours preceding the discovery of their loss),

this is the opinion of Rabbi Eliezer ben Shamua.

Rabbi Yehuda says

at three points in time

wine [that had been set aside as maaser for tevel elsewhere] must be checked [to ensure it has not turned to vinegar]:

at [the time of] the east wind after Succos,

when the [grape] buds emerge,

and at the time that liquid appears in the unripened grapes.

or sends another messenger for him,