As other answers indicated, if you have an agreement with your agent THAT DEFINES COMMISSIONS, your agent has a case. In general practice, most agents accept the fact that commissions sometimes get negotiated down in a short sale and live with it to get the deal done.
It is an unpleasant reality in working with short sales that this sometimes happens.
If you do not have an agreement that defines commissions, you have no obligation -- unless you feel a moral one.
1) Agent agrees to work for whatever the advertised commission is.
2) Agent works on retainer
3) Agent works for a specified commission rate regardless of co-operating commission offered
These elements are usually agreed upon when you hired your agent and may have been renegotiated during the specific transaction.
If there was no contract, then what you choose to do above and beyond for your agent is entirely up to you. Though your agent may also have to follow his/her company policy.