In the Vancouver Washington market probably about 50 percent of earnest money agreements ask for a seller to contribute to the buyers closing costs.  Often an astute seller will recognize that they are paying commission on these costs and ask that commission be paid on the selling price minus these costs.  This has always seem reasonable to me and as a buyers agent I have never seen fit to object when asked to agree to this.  (usually it means a reduction of something like 20 dollars in my net commission.)

However too many agents in my market are requesting that this change be made by adding the language as an addendum to the earnest money agreement.  In my opinion this is a real mistake and reveals an agents lack of understanding of the legal boundaries of the earnest money agreement.  Buyer's agents are not party to the earnest money agreement.  The earnest money agreement is an agreement between a potential buyer and the seller of a property.  Buyer's agents don't have to sign this agreement (except on the LBP addendum--by federal law). 

If a seller and a listing agent want to reduce the sales price on which commission is paid, it cannot be done on an earnest money agreement.  It must be done on a contract separate from the EM agreement.  In my opinion you should send the buyer's agent a separate note to sign stating the new sales price used for commission.  When a listing agents asks for this change as part of the EM agreement it lacks professionalism and worse reveals to the potential buyers that the listing agent does not understand EM agreements.