I listed my house with an agent with the understanding that if one of two friends buy my property, she would b?

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. 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. Is this ethical? Or legal?