professional ethics dilemma / scenario

hall5456

I was hoping forum users might be able to help with this query I had. I work as an administrator in a university and a professor approached me offered the following deal:

1. he has a relatives in Australia he is planning to visit anyway
2. but also, he could collect data for research that the department might benefit from in terms of publications (maybe 4/5 days)
3. The remaining time would be with his family (15 days) so will be taking some annual leave
4. He would like me to authorize that the department should / could pay some or half the department

Something similar is happening where a colleague is going abroad to visit family, and is being asked to do some work for the department on behalf of the university while on annual leave, and the department will pay for their flight or half their flight.

What are the professional ethical guidelines and advice on this, as I believe their are several self-interest motives in both scenarios, and I am unclear how best to deal with both situations to avoid upsetting those involved.

thank you in advance