What is the ruling on saying: "By divorce, she will not return to her [the sponsor]," with the intention of transferring the maid's sponsorship and avoiding problems, then wishing to keep her in response to the father's request, and does divorce occur by that?

The majority of scholars hold that a conditional divorce takes effect upon the fulfillment of the condition, and this is the preponderant opinion. If you swore by divorce that the maid would not return to your grandmother's house, and she did return, then the divorce has occurred according to the majority of scholars. You may take back your wife before her 'iddah (waiting period) is complete, provided this divorce does not complete the three divorces. Shaykh al-Islam Ibn Taymiyyah differed, saying that an expiation for an oath (kaffarat yamin) is required if divorce was not intended, but the preponderant view is that of the majority.