KöMaL 2023 K784

Let us replace the letters below with digits $0$-$9$ (with the exception of one digit) such that the result of the subtraction of the two 3-digit numbers is the closest possible to $300$: $ABC - DEF = GHJ.$ Prove that the result that has the smallest difference from $300$ can only be obtained in an unique way. (Different letters has to be replaced with different digits.)