Responsa for Challah 3:8

הַנּוֹטֵל שְׂאֹר מֵעִסָּה שֶׁלֹּא הוּרְמָה חַלָּתָהּ וְנוֹתֵן לְתוֹךְ עִסָּה שֶׁהוּרְמָה חַלָּתָהּ, אִם יֶשׁ לוֹ פַרְנָסָה מִמָּקוֹם אַחֵר, מוֹצִיא לְפִי חֶשְׁבּוֹן. וְאִם לָאו, מוֹצִיא חַלָּה אַחַת עַל הַכֹּל:

One who takes leaven out of dough from which <i>Challah</i> had not been removed and puts it into dough from which <i>Challah</i> had been removed: if one has a supply [of dough] from another place, one can take out [<i>Challah</i>] in accordance with the precise amount; but if one does not, one takes out one [portion of] <i>Challah</i> for the whole [dough].

Teshuvot Maharam