R' Eliezer says: [Challah] may be taken from a tahor [dough] on behalf of a tamei [dough]. How? [If one has] a tahor dough and a tamei dough, he takes an amount sufficient for challah from the dough whose challah has not been taken, and places less than a beitzah [of dough] in the middle, so that he will be taking [challah] from something in proximity [to the doughs]. But the Sages forbid [it].

One may eat casually from dough until it is thoroughly mixed in the case of wheat [dough] or [until] it is packed in the case of barley [dough]. [Once] she thoroughly mixed wheat [dough] or packed barley [dough], one who eats from it is liable to death.  Once she puts in the water, she should separate her challah, provided that there are not five reva'im of flour there.

[If] her dough became dimua before she thoroughly mixed [it], [it] is exempt [from challah], for a dimua mixture is exempt [from challah]. But [if the dough became dimua] after she thoroughly mixed [it], it is obligated [in challah].  [If] there arose in [the dough] an uncertainty of tumah before it was thoroughly mixed, it may be made in tumah; but if after she thoroughly mixed [the dough], it must be made in taharah.

[If] she consecrated her dough before she thoroughly mixed [it], and she redeemed it, it is obligated [in challah]. [If she consecrated the dough] after she thoroughly mixed [it] and she redeemed it, it is obligated [in challah]. [But if] she consecrated it before she thoroughly mixed [it], and the [Temple] treasurer mixed it, and later she redeemed it, it is exempt [from challah], because at the time of its [potential] liability it was exempt.