What do I need to do?

My family and I are moving to a new place in Georgia. So I have decided to sell our old house. I listed my house with an agent with the understanding that if one of two friends buy my property, she would be compensated at 3% commission. One of my friends has made an offer. I was surprised when my agent sent me the estimated pay out from the transaction; she put in her commission as 6%. Her explanation is that the original deal was only good until she listed the house in MLS. What is MLS? What do I need to do?