Someone was making a dough and in the process a large piece fell out from the mixture onto the floor. They had made exactly the right amount in order to take challah with a bracha. The part that fell onto the floor they now no longer want to use as part of the dough. Therefore the mixture is now missing a proper “shiur” (amount) in order to take challah with a bracha. Can a bracha still be made when challah is taken from the dough in such a case.
Assuming that the the part that fell on the floor is not actually dirty, and can still be eaten, it appears that if it is mixed together with the rest of the dough it will still be considered a shiur for challah.
The fact that the person separating the dough doesn’t want it will not render the mixture separated, because he might still decide to eat it, or for his children etc. to eat it.
However, if the part that fell is no longer edible by normal standards, it will not be considered part of the dough for purposes of challah.