Vaishnava Samhita. Sakshi Gopala Charitam. Vol III Book VI Chp 23 Shloka 13

अप्रार्थितोपि वृद्धस्तु वटुना वारितोपि च  |
दास्यामि कन्यामिन्येव प्रतिजज्ञे पुरो हरेः ||

Brahmachari did not request him to do so. In fact he cautioned the old man not to make a promise to that effect. But the old man insisted on that promise in the presence of Lord Shri Krishna.