Are you the Seller of the property? I'm assuming you have a real estate agent, because you mention the MLS number of the property. Your agent may have already put the change regarding the Buyer incentive you're offering into the Multiple Listing Service. You can ask your agent to show you an updated copy of your MLS listing, verifying the change.
Listings changes can go through an automatic "feed" to Trulia from your local MLS. Sometimes delays happen there. If a reasonable amount of time has passed and the updates still aren't on Trulia, tell your agent about it. He/she can input the changes manually, and can also notify Trulia of the problem. Good luck with your sale.
Maggie Hawk, REALTOR
Watson Realty Corp.