How to make a Roblox currency system (with AI)

Example output from a build like this

4 files in this example. Picoo writes this code for your prompt, so file names and sizes vary.

Sample output: ServerScriptService/CurrencyManager.lua

--!strict
-- ServerScriptService/CurrencyManager.lua  (Picoo · excerpt)
local Players = game:GetService("Players")
local RunService = game:GetService("RunService")
local SaveService = require(script.Parent.CurrencySaveService)
local Logger = require(script.Parent.CurrencyLogger)

type Currencies = { cash: number, gems: number, premium: number }

local balances: { [Player]: Currencies } = {}
local leaderstatsMap: { [Player]: { [string]: NumberValue } } = {}
-- true ONLY after a load that succeeded. A failed load is never saved over:
-- saving the defaults would replace the player's real balances with them.
local loaded: { [Player]: boolean } = {}
-- A Studio play test of an unpublished place fails every load: it plays on the
-- defaults and is never saved. A live server refuses changes after a failed load.
local playTest: { [Player]: boolean } = {}

local function setupLeaderstats(player: Player, c: Currencies)
    local ls = Instance.new("Folder"); ls.Name = "leaderstats"; ls.Parent = player
    local map: { [string]: NumberValue } = {}
    for currencyName, value in pairs(c) do
        local nv = Instance.new("NumberValue")
        nv.Name = currencyName:gsub("^%l", string.upper)
        nv.Value = value; nv.Parent = ls
        map[currencyName] = nv
    end
    leaderstatsMap[player] = map
end

Players.PlayerAdded:Connect(function(player)
    -- ok is false when the DataStore call failed; saved is nil for a new player.
    local ok, saved = SaveService:Get(player)
    loaded[player] = ok
    playTest[player] = not ok and RunService:IsStudio()
    if not ok then warn("[Currency] load failed for " .. player.Name .. " (this session will not be saved)") end
    saved = saved or { cash = 100, gems = 0, premium = 0 }
    balances[player] = saved
    setupLeaderstats(player, saved)
end)

local CurrencyManager = {}

-- Returns true only when the balance changed, so a Robux receipt handler can
-- return PurchaseGranted on true and NotProcessedYet otherwise.
function CurrencyManager:add(player: Player, currency: string, amount: number, reason: string): boolean
    local b = balances[player]
    if not b or not (loaded[player] or playTest[player]) or amount <= 0 then return false end
    b[currency] = (b[currency] or 0) + amount
    leaderstatsMap[player][currency].Value = b[currency]
    if loaded[player] then SaveService:Set(player, b) end -- a play test never saves
    Logger:write(player, currency, amount, reason)
    return true
end

function CurrencyManager:spend(player: Player, currency: string, amount: number, reason: string): boolean
    local b = balances[player]
    if not b or not (loaded[player] or playTest[player]) or amount <= 0 then return false end
    if (b[currency] or 0) < amount then return false end
    b[currency] -= amount
    leaderstatsMap[player][currency].Value = b[currency]
    if loaded[player] then SaveService:Set(player, b) end -- a play test never saves
    Logger:write(player, currency, -amount, reason)
    return true
end

return CurrencyManager