Before showing the property, the realtor should have provided you with a one day listing agreement, if he/she did not, then frankly, you are not obligated nor is the tenant.
However, for his/her efforts, I would advocate a 5% commission based upon the rental price. So if the monthly rental is $1000 per month, your cost is $500, to which the agent will most likely get half. In the spirit of good karma, I would offer same. Understand that on a rental a Realtors busts his/her butt for pennies when u consider the time and paperwork. I believe it would be in your best interests to ensure that at least someone is represented by an agency.
Hope al work out well.
Francesca Patrizio, Broker Sales Associate, ePro, SRES
Gloria Nilson Realtors, Real Living