Jamieson, Fausset and Brown on 1 Corinthians 7:11

11. But and if she depart--or "be separated." If the sin of separation has been committed, that of a new marriage is not to be added (Mt 5:32).

be reconciled--by appeasing her husband's displeasure, and recovering his good will.

let not ... husband put away ... wife--In Mt 5:32 the only exception allowed is, "saving for the cause of fornication."