Rather than saying:
Would you like to pay me to do some modeling
and see what happens?
how about asking:
What you be willing to pay for some reports of
flaws in your design?
Or:
Would you like to pay me to generate ten crazy
scenarios you haven’t considered?
Hmm, upon reflection I don't think any manager would be interested in receiving "crazy" scenarios.
These are probably better approaches:
Would you be interested in an identification of flaws
in your design?
Would it be useful to have ten use cases that illustrate
undesirable behavior that you haven’t yet considered?
If you present this to a manager, you are going to have to convince the manager that this is not only worth doing, but of higher priority than other tasks on his/her plate. Also, you will have to give some evidence to support the claim that you can do what you claim.