John Gill Commentary Genesis 34:15

"Only on this condition will we consent unto you: if ye will be as we are, that every male of you be circumcised;" — Genesis 34:15 (ASV)

But in this will we consent to you
Upon the following condition, that Dinah should be given in marriage: if you will be as we [be], that every male of you be circumcised ;
as the sons of Jacob were, according to the command given to Abraham their great grandfather, (Genesis 17:10) .