Idaho State Police says Douglas Anderson bludgeoned 78-year-old Darole Carpenter and repeatedly stabbed him in the stomach in July.
According to court records, Anderson pleaded guilty to second degree murder and theft by false promise.
Investigators say they found Carpenter's blood in Anderson's vehicle and have video evidence that the vehicle was at the house at the time of the murder.
Police say they believe Anderson killed Carpenter to try to hide that he had defrauded him of about $30,000. Officials say Anderson had promised to use the money to buy cattle and hay for Carpenter but never went through with the transaction.
Investigators say Anderson also forged his wife's signature on a promissory note for $230,000 in which he again promised to buy cattle and hay.
Anderson is scheduled to be sentenced March 25.