There are few questions here:
Ask for proof? Sure, anyone can ask for "proof" but what will be produced, if anything, will mean different things to different people. In representing a seller there is no item that I would be willing to advise my sellers to produce that could prove that another offer existed.
Showing the competing offer would greatly reduce the seller's position of power in negotiations. Your lack of knowledge of the terms and price of the competing offer is what drives competition in multiple offer situations and therefore greatly benefits the seller. Cluing you in to the substance of the other offer would mean that you could tailor your offer to be just a little bit better when you may have been willing to be a lot better.
A signature page, preapproval letter, or the name of the competing buyer's agent, etc. proves very little of the existence of another offer. In writing your question you already doubt the honesty of the agent. It would follow that if you were correct in assessing their integrity that it would also be possible for them to lie about the name of the agent, etc, or produce false or outdated documentation. I'm not much into conspiracy theories though so I generally don't bother with this sort of posturing; it really will only serve to insult the listing agent which will hurt your possibility of acceptance in the end.
I like where Jennifer is going with using the code, but I think it simpler than that. Article 1 of the 2011 Realtor's Code of Ethics states:
When representing a buyer, seller, landlord, tenant, or other client as an agent, REALTORSÂ® pledge themselves to protect and promote the interests of their client. This obligation to the client is primary, but it does not relieve REALTORSÂ® of their obligation to treat all parties honestly...
There it is, plain and simple; realtors can't lie in a real estate transaction. If you really want to go through the exercise of finding out whether another offer exists, file a complaint with the local board of realtors against said lying realtor (I'm serious about this by the way) and ask them to produce proof that they didn't lie. Any agent worth their salt will have copies of both offers and will quickly be able to put both on the table in front of the hearing panel. One of two things will happen, they will be able to produce both or they risk losing an ethics complaint which could carry a fine. Be aware however that this sort of strategy only really works after the fact and if employed during negotiations will likely cause you to win the battle but lose the house.
What would I do? Trust is the basis of all working relationships, and if you donâ€™t trust your realtor you need to find a new one. Complicating the matter however is the fact that if you submitted an offer, a contract for representation likely exists between you and their broker. You could ask to be assigned to another realtor at the same brokerage or you simply ask the realtor to voluntarily end the contract for representation. Notice the key word in the last sentence though â€“ voluntarily. You might even offer them some money for the time they spent and in consideration of their willingness to cancel the contract. The compensation might soften the blow of losing a client and make it easier for them and you to move on.
Now I know what you are thinking (or at least could be thinking). Why would I give a dime of my hard earned money to that lying cheating jerk? By no means do you have toâ€¦but then again they donâ€™t have to agree to cancel their contract either. Failing their agreement to cancel or the brokerâ€™s reassignment to another agent, one of two scenarios plays out. One: you buy a house with a person you donâ€™t trust and question whether you got hoodwinked for as long as you own the house. Two: you put your plans on hold until the contract expires and you find another agent at that time. The choice is ultimately up to you.
Confront them? ABSOLUTELY. I tend to believe that people are generally good and most real estate agents donâ€™t make a habit of lying to their clients. It is often said that there are three versions of every story. In this case there is your version, their version, and the truth. If you give your agent the chance to explain themselves and tell the whole story, I am betting you will both end up believing in the third version of the story â€“ the truth.
Note: If it does turn out that they are lying and cheating scum, because those agents do exist as well. I want you to call me so that I can help you file an ethics complaint against them so they donâ€™t have the opportunity to do this to another consumer.
Just go to someone else? PROCEED WITH CAUTION!! If you have a representation agreement with your current agent, working with another agent may obligate you to pay the agreed upon compensation to your old agent as well as your new agent.