If someone finds "a large sum" of money on the ground, they are REQUIRED to turn it over to the police. If it goes unclaimed, they can keep it, but you just can't keep it just because you don't know who it belongs to.
Penal Code §485 states that:
One who finds lost property under circumstances which give him knowledge of or means of inquiry as to the true owner, and who appropriates such property to his own use, or to the use of another person not entitled thereto, without first making reasonable and just efforts to find the owner and to restore the property to him, is guilty of theft.
Further, Civil Code §2080 states that:
Any person who finds a thing lost is not bound to take charge of it, unless the person is otherwise required to do so by contract or law, but when the person does take charge of it he or she is thenceforward a depositary for the owner, with the rights and obligations of a depositary for hire. Any person or any public or private entity that finds and takes possession of any money, goods, things in action, or other personal property, or saves any domestic animal from harm, neglect, drowning, or starvation, shall, within a reasonable time, inform the owner, if known, and make restitution without compensation, except a reasonable charge for saving and taking care of the property.
Still further, Civil Code §2080.1(a) states that:
If the owner is unknown or has not claimed the property, the person saving or finding the property shall, if the property is of the value of one hundred dollars ($100) or more, within a reasonable time turn the property over to the police department of the city or city and county, if found therein, or to the sheriff’s department of the county if found outside of city limits, and shall make an affidavit, stating when and where he or she found or saved the property, particularly describing it.
So while the "true owner" should be entitled to getting restitution of the money (you would won't the same if it were your money, right?), being charged with grand theft is a stretch. You need to find a good lawyer to fight this case.