Valmiki Ramayana Ayodhya Kanda Sarga 23 Shloka 9
Original Shlokaयदि नैवं व्यवसितं स्याद्धि प्रागेव राघव।।2.23.9।। तयोः प्रागेव दत्तश्च स्याद्वरः प्रकृतश्च सः।
1 source checked for this shloka
राघव O Rama, प्रागेव even before, तयोः their own, व्यवसितम् decision, एवम् in this way, न स्याद्धि if it was not like that, प्रकृतः by nature, सः that, वरः boon, प्रागेव दत्तः स्यात् would have been given long before.
If both of them had not decided before, O Rama the boon granted to Kaikeyi naturally would have been fulfilled long ago.