One piece of advice on this:

DON'T DO IT!!!

I'll give you a real-life example to illustrate why:

A friend and colleague of mine once had a listing on a $4,000,000.00 piece of property. He had the listing for a few months with very little activity on it. He approached one of his investors about making an offer on the property, and the investor submitted a $2,500,000.00 offer to purchase the property. The listing agent then contacted the seller to discuss the offer with him. After a discussion that lasted for several minutes it was decided that the listing agent would draft a counter-offer in the amount of $2,750,000.00. Since the seller was out of town on vacation at the time he verbally authorized the agent to sign the counter-offer himself as the authorized agent for the seller. The counter-offer was then signed by the agent and submitted to the prospective buyer. The buyer then signed his acceptance of the $2,750,000.00 counter-offer.

Unbeknownst to the listing agent another agent had submitted the property to a private educational institution that was desperately in need of locating a site to build a new school. The educational institution decided that they could not afford to let this site get away from them, and they immediately offered the full $4,000,000.00 asking price for the property.

When the seller received the $4,000,000.00 offer for his property he immediately exclaimed that he never authorized the listing agent to sign the $2,750,000.00 counter-offer on his behalf.

Fortunately for the agent, and partially due to the agent's prior relationship with the first prospective buyer, this buyer decided not to sue and walked away from the deal. However, should you decide to sign counter-offers on behalf of your own clients in the future, you may not be so fortunate.