After hours of experimenting the code finally works
The problem was the hash value. It was always empty so I put the hash value directly as a argument.

For those who want to know how to add cash to the player, here's an example:

public void moneyfunction ( ){

    if (Game.IsKeyDown(Keys.Left)) {
            NativePointer nativePointer = new NativePointer();
            int money = 0;
            NativeFunction.Natives.STAT_GET_INT(0x324C31D, nativePointer, -1);            // SP0_TATAL_CASH = 0x324C31D Michael's Cash
            nativePointer.SetValue(nativePointer.GetValue<int>() + 2000);                         // Cash value + 2000
            NativeFunction.Natives.STAT_SET_INT<int>(0x324C31D, nativePointer.GetValue<int>(), -1);      // Add 2000$
        }

}

I know this code needs to be optimized but it will do for now

Thank you very much for your help, without your help I would never find it out

@Jitnaught Unfortunately I don't know how to use NativeArgument. I hope someone in the forums has an idea.

@Dreamlncode The method "BOOL STAT_GET_INT(Hash statHash, int *outValue, int p2)" expects an "int *outValue". What is the *outValue?

@Jitnaught I searched in Native DB and found these methods about STAT:

BOOL STAT_GET_INT(Hash statHash, int *outValue, int p2)
BOOL STAT_SET_INT(Hash statName, int value, BOOL save)

That example below is just to get the cash value. I'll try to set the value after the get method works:

public void moneyfunction (){

    if (Game.IsKeyDown(Keys.Left)) {
            int* val;
            Hash hash = NativeFunction.Natives.GET_HASH_KEY("SP0_TOTAL_CASH");
            NativeFunction.Natives.STAT_GET_INT(hash, &val, -1);

            Game.DisplayNotification("Money: " + &val);

        }

    }

Something is wrong with the second parameter (int* val) . It seems to be a pointer but I have no idea how to handle pointers. The compiler doesn't accept that syntax I tried
I think that's the last missing puzzle piece

I tried your code but what do I need to make it run? I fit the code a little bit to make the compiler accept it.
There still is one thing the compiler is moaning about: "Hash.SET_PED_MONEY".
I guess a using-directive is missing or something else.

public void moneyFunction (){

    if (Game.IsKeyDown(Keys.Left)) {
            Ped player = Game.LocalPlayer.Character;
            int PlayerMoney = NativeFunction.Natives.CallByHash<int>(**Hash**.GET_PED_MONEY, player);
            NativeFunction.Natives.Call(**Hash**.Natives.SET_PED_MONEY, player, (PlayerMoney -= 50));
            Game.DisplayNotification("You have been charged with $50");
        }

    }

Do you have any idea what is wrong?

@Miawwe Thank you for replying. I already tried Game.LocalPlayer.Character.Money but it doesn't work. When I use the Game.DisplayNotification the Money Value shows me 0 although my character has over 700k for example.
GTA.Player.Money - GTA.Player.Money(40); doesn't work either. Can it be that this works just in C++? I'm using C# and the ..Character.Money value seems to be useless.

I'm working on a mod where cars consume fuel and where cars can be refueled on gas stations. My problem is I don't know how to make the player pay for refueling. It's boring to get the fuel for free.
Does anybody know the function or method in C# to take money from the player?