What is the religious ruling regarding a wife who requests divorce from her current husband, after a marriage based on love, in order to obtain custody of her children from a previous marriage, knowing that she had previously willingly relinquished their custody?

A woman is not permitted to seek a divorce or khul’ unless there is a legitimate religious excuse. If she requests custody to harm her ex-husband or anyone else, or out of spite, then it is forbidden for her to do so.

If she waives her custody of her children in exchange for a divorce, this condition is valid and she is bound by it. It is not permissible for her, after obtaining the divorce, to return and take the children. This is the madhhab of Imam Malik, and it is supported by the principle that conditions are originally permissible and binding unless forbidden by the Sharia.