Everything is not as black and white as you may think.. And of course there will always be vulnerable people and some evil people will always take advantage of them.. Anyways if we look beyond that and if two people agree to marry in a consenting arrangement then I don't see the issue
Of course, consent between equal minds and independent people, is nobody's business. If this Shiah sounding arrangement is even halal.
My issue is the following scenario:
Imagine you are a poor father/mother and some rich man or diaspora dayuus, comes to your daughter. Behind your back convinces her to waive her rights, in exchange for sex and occasional money? You would be heartbroken and outraged!
If the man dared ask your permission for qudbo siro, you'd probably beat him to death.