What happens to a partial *Istibra'* period when ownership of the female slave transfers before completion?

Primary text

If part of the *Istibra'* period has passed while she was under the ownership of the seller, she is obligated to complete the remainder of the period after she is freed. The obligation to complete the *Istibra'* is not interrupted by the transfer of ownership because she did not become permissible sexual property (*firash*) for the second owner, nor was *Istibra'* made obligatory upon her by the act of his freeing her.