Two buyers one house one realtor

[Sharita S.]

1. Found myself with a  tenant and no house.  I called a random realtor who showed us two houses.  I called about a 3rd property and she said that she was already on the way to show it to someone else.  I asked if she could show it to us as well and she agreed.  Upon arrival she made it clear that she would be writing up an offer for the other client.  I told her that I was unsure of he ability to represent both parties.  She basically stated that she was sticking with her first buyer.  I have now contacted another realtor to write up an offer.  Is it reasonable/ethical to negotiate part of his commisssion seeing that his only time invested is writing the contract?