Generally, the short sale lender covers this expense --- but the short sale lender must approve the price, terms and conditions based on what the investor guidelines.
Some loans -- Fannie Mae and Freddie Mac --- are not permitted to reduce the agreed upon sales commissions. Others may not be so lucky.
Since you identify yourself as the buyer, this may not be your concern. What you should be concerned about is if you are asked to make a contribution towards the short sale if what the short sale lender approves is higher than your purchase price. The easiest solution to meeting that approved price is to raise the offer, or negotiate again and try to meet the lender's price somewhere in between.