new assist[MAX_PLAYERS] = INVALID_PLAYER_ID;

public OnPlayerGiveDamage(playerid, damageid, Float:amount, weaponid, bodypart)
{
       if(GetPlayerTeam(playerid) != GetPlayerTeam(damageid)) return 0;

      assist[damageid] = playerid;
      return 1;
}

public OnPlayerDeath(playerid, killerid, reason)
{
      if(assist[playerid] != killerid && assist[playerid] != INVALID_PLAYER_ID)
      {
            SendClientMessage(assist[playerid], -1, "+500$ for assist");
            assist[playerid] = INVALID_PLAYER_ID;
      }
      return 1;
}