An ethical dilemma

My friend Jane is a freelance trainer and was approached, recently, by an agency who offered her a short term teaching course at a Midlands college for (say) 20 GBP per hour. When she got to the college, the Dean told her that he had not engaged the agency to fill the post, but had advertised it on Monster for (say) 40 GBP per hour. The agency had offered to provide a tutor for (say) 35 GBP per hour, so the Dean accepted.

The Dean went on to say that he was not keen on using agencies, and had other work for Jane to do, all at 40 GBP per hour, when the short term course was over.

So, the college had no contractual obligation to the agency, and Jane's commitment was for the short term course alone, at the reduced rate of 20 GBP per hour. Should she complete the course and then accept further work direct from the college at twice the money she was getting from the agency?

What would you advise?

PKP