Charles Ellicott Commentary Genesis 29:27

"Fulfil the week of this one, and we will give thee the other also for the service which thou shalt serve with me yet seven other years." — Genesis 29:27 (ASV)

Fulfill her week. —The marriage festival seems to have lasted a week, as was the custom in later times (Judges 14:12), and to have forsaken Leah during this period would have been to offer her an insult which her brothers must have avenged. Appeased, therefore, by the promise of Rachel as soon as the seven days are over, Jacob, rather than quarrel with the whole family, submits to the wrong.

The Hebrew is remarkable, “Fulfill the week of this, and we will give to you also the this for the service.” But in Hebrew this ... this means the one and the other (Genesis 31:38; Genesis 31:41), and it is a mistake to suppose that the language will allow the first this to be understood of any one but Leah, and the second this of any one but Rachel.