John Gill Commentary Genesis 24:36

"And Sarah my master`s wife bare a son to my master when she was old. And unto him hath he given all that he hath." — Genesis 24:36 (ASV)

And Sarah, my master's wife Who must be well known to this family, by name at least, being, as is generally supposed, the sister of Milcah, Nahor's wife, and Bethuel's mother;

the same bore a son to my master when she was old; even when ninety years of age, (Genesis 17:17); this is mentioned because it was an extraordinary thing, and an instance of divine power and of the favour of God; and this son of their old age must be very dear to Abraham, as it follows:

and to him has he given all that he has; by his will: Jarchi says, the servant had now the schedule of all in his hands, which he showed; but this is mere conjecture.