new PlayerCash[MAX_PLAYERS];

new String[30];
format(String,sizeof(String),"Your Cash: %d",PlayerCash[playerid]);
SendClientMessage(playerid,COLOR,String);

if(PlayerCash[playerid] >= 500)
{
//give the player the m4 or do whatever because the players cash is above 500
PlayerCash[playerid] -= 500;//this will take the 500 from the players cash for purchasing the M4.
}
else
{
SendClientMessage(playerid,COLOR,"You do not have enough cash to buy a M4!");
}

stock MoneyPlus(playerid, amount)
{
  PlayerCash[playerid]+=amount;
}
stock MoneyMinus(playerid, amount)
{
  if(PlayerCash[playerid]-amount>=0)
  {
    PlayerCash[playerid]-=amount;
    return 1;
  }
  return 0;
}

// top
new
	pCash[MAX_PLAYERS];

#define GiveUserMoney(%1,%2) pCash[%1] += %2
#define GetUserMoney(%1) pCash[%1]
#define ResetUserMoney(%1) pCash[%1] = 0

if(GetUserMoney(playerid) < 200) return SendClientMessage(playerid, COLOR, "You dont have the cash");
GiveUserMoney(playerid, -200); // Remove 200 from the cash of the player

// top
#define ITEM_NONE 0
#define ITEM_CONDOM 1
#define ITEM_DRUGS 2
#define ITEM_DRINKS 3
#define ITEM_PHONE 4
// more items here

#define MAX_ITEMS 5 // increase if you use more items
#define MAX_ITEMSLOT 3 // amount of items player can carry, remember you need to save each item
#define MAX_ITEMAMOUNT 20 // the max amount of an item a player can carry

new
	pItem[MAX_PLAYERS][MAX_ITEMSLOT],
	pItemAmount[MAX_PLAYERS][MAX_ITEMSLOT];


stock GiveUserItem(playerid, item, amount)
{
	/*
		returns 0 if NOT successfull
		returns 1 if successfull
	*/
	new
		slot = MAX_ITEMSLOT;
	for(new i = 0; i < MAX_ITEMSLOT; i++)
	{
		if(pItem[playerid][i] == ITEM_NONE || pItem[playerid][i] == item)
		{
			slot = i;
			break;	
		}
	}
	if(slot == MAX_ITEMSLOT) return 0;
	if((pItemAmount[playerid][slot]+amount) > MAX_ITEMAMOUNT) return 0;
	pItem[playerid][slot] = item;
	pItemAmount[playerid][slot] += amount;
	if(pItemAmount[playerid][slot] <= 0) pItem[playerid][slot] = ITEM_NONE;
	return 1;	
}

stock GetUserItem(playerid, item)
{
	/*
		returns the amount of an item
	*/
	for(new i = 0; i < MAX_ITEMSLOT; i++)
	{
		if(pItem[playerid][i] == item) return pItemAmount[playerid][i];			
	}
	return 0;
}

stock GetItem(item)
{
	new
		string[30];
	switch(item)
	{
		case ITEM_CONDOM: format(string, sizeof(string), "Condom");
		case ITEM_DRUGS: format(string, sizeof(string), "Drogen");
		case ITEM_DRINKS: format(string, sizeof(string), "Drinks");
		case ITEM_PHONE: format(string, sizeof(string), "Phone");
		// Add more items here
		default: format(string, sizeof(string), "Nothing");
	}
	return string;
}

// buy command
new
	item, 
	amount,
	string[80];

if(sscanf(params, "dd", item,amount)) // check the parameters with sscanf or strtok
{
	SendClientMessage(playerid, COLOR, "USAGE: /buy [item] [amount]");
	format(string, sizeof(string), "Available Items -> Phone:%d | Condom:%d ", ITEM_PHONE, ITEM_CONDOM);
	SendClientMessage(playerid, COLOR, string);
	return 1;
} 
if(amount <= 0) return SendClientMessage(playerid, COLOR, " Wrong amount");
if(item != ITEM_PHONE && item != ITEM_CONDOM) return SendClientMessage(playerid, COLOR, "You cant bought this here");

new
	bool:success = GiveUserItem(playerid, item, amount);
if(!success) return SendClientMessage(playerid, COLOR, "Check your bag, you cant put this item in the bag right now"); 

format(string, sizeof(string), "You bought %d %s", amount, GetItem(item));
SendClientMessage(playerid, COLOR, string);


// in the call command you could then check

if(GetUserItem(playerid, ITEM_PHONE) == 0) return SendClientMessage(playerid, COLOR, "You dont have a phone");