The creature, upon realizing his horrific existence and being abandoned by Victor, instead demands Victor create a companion for him. He argues that isolation and loneliness are unbearable, and having another creature like himself would alleviate the pain of his existence.
Victor initially agrees to this request, but ultimately decides against it out of fear that the two creatures could procreate and create a race of monsters that would threaten humanity.
Therefore, there is no "deal" made, but rather a demand from the creature that Victor initially agrees to, but later breaks.