How to make a button, that you can click only once after you purchased the game pass?

I suggest using DataStoreService. Create a Script inside ServerScriptService and the code should be looking like this:

local DSS = game:GetService("DataStoreService")
local DS = DSS:GetDataStore("GamePassOwners")

local function create(plr)
    local userId = plr.UserId
    local key = "Player_" .. userId

    local value = Instance.new("BoolValue", plr)
    value.Name = "HasGamePassOn"

    local ls = Instance.new("Folder", plr)
    ls.Name = "leaderstats"

    local money = Instance.new("IntValue", ls)
    money.Name = "Money"

    local data
    local success, result = pcall(function()
        data = DS:GetAsync(userId)
    end)

    if success and data then
        value.Value = data[1] or false
        money.Value = data[2] or 0
    else
        print("ERROR!" .. result)
    end
end

local function save(plr)
    local userId = plr.UserId
    local key = "Player_" .. userId

    local tableToSave = {
        plr.HasGamePassOn.Value,
        plr.leaderstats.Money.Value
    }

    local success, result = pcall(function()
        DS:SetAsync(userId, tableToSave)
    end)
    if success then
        print("Saved!")
    else
        print("Error")
        warn(result)
    end
end

game.Players.PlayerAdded:Connect(create)
game.Players.PlayerRemoving:Connect(save)
game:BindToClose(function()
    if game:GetService("RunService"):IsStudio() then return end

    for _, plr in pairs(game.Players:GetPlayers) do
        save(plr)
    end
end)

Then in the gamepass script, it should be like this:

--Must be inside the button and in a LocalScript
local mps = game:GetService("MarketplaceService")
local plr = game.Players.LocalPlayer
local id = 0000000

script.Parent.MouseButton1Click:Connect(function()
    if game:GetService("MarketplaceService"):UserOwnsGamePassAsync(plr.UserID, id) then
        if plr.HasGamePassOn.Value == false then
            print("Player owns gamepass.")
            local points = plr:FindFirstChild("leaderstats")
            local money = points:FindFirstChild("Money")
            money.Value = money.Value + 1
            plr.HasGamePassOn.Value = true
        else
            print("You already clicked.")
        end
    else
        mps:PromptPurchase(plr, id)
    end
end)

If there are errors or mistakes, please let me know! ^^

I don't quite get what you mean but I cooked something up that might work!

Also by the way, You were applying Money to the client, That won't update to the server so if you try and use that cash, it won't work cause it's client sided!

Add a RemoteEvent to replicatedStorage

Local Script:

local PlayersService = game:GetService("Players")
local MarketPlaceService = game:GetService("MarketplaceService")
local replicatedStorage = game:GetService("ReplicatedStorage")

local player = PlayersService.LocalPlayer

local addMoneyRE = replicatedStorage:FindFirstChild("AddMoney")

local button = script.Parent

local gamePassID = 1 -- Change to gamepass id
local ownerId = 15324 -- Change to your UserId

local fired = false

button.MouseButton1Click:Connect(function()
    if player.UserId ~= ownerId then
        if MarketPlaceService:UserOwnsGamePassAsync(player.UserId, gamePassID) and fired == false then

            addMoneyRE:FireServer(1) -- Change to amount you wanna give

            print(player.Name.." owns item!")

            fired = true

        else
            MarketPlaceService:PromptGamePassPurchase(player, gamePassID)

        end 
    else
        addMoneyRE:FireServer(1) -- Change to amount you wanna give

    end
end)

Server Script:

local replicatedStorage = game:GetService("ReplicatedStorage")

replicatedStorage:FindFirstChild("AddMoney").OnServerEvent:Connect(function(player, amount)

    player.leaderstats.Cash.Value += amount

end)