From: Rabbi Moshe Sofer
To: Rabbi Eliezer of Dreznitz
The Chasam Sofer rules that both the agent and the homeowner may seek annulment of a separation of challah or terumah, but through distinct mechanisms: the agent's annulment uproots his act retroactively as if never performed, reverting the produce to tevel status, while the homeowner's annulment nullifies the agency itself ab initio. He further rules that annulment of the separation does not render the original blessing a berachah levatalah, since the mitzvah of separating challah and terumah inherently encompasses the possibility of future annulment.
Source: Shu"t Chasam Sofer YD §320