Buy cash gui wont add cash when bought?

Locally this works. In an actual server this won't work because you can't use ProcessReceipt through a localscript. You should be prompting the purchase through the client and processing it through the server.

This wiki article has a nice ProcessReceipt example.

This one is also helpful.

Oh, and this one.

Here's a Filtering Enabled compatible version of your code.

Script in ServerScriptService

local MarketplaceService = game:GetService("MarketplaceService")
local ProductID = 92041867

MarketplaceService.ProcessReceipt = function(info)

    -- All of this code is a slighly modified version of the code on the wiki.
    local player = game:GetService("Players"):GetPlayerByUserId(info.PlayerId)
    if not player then
        return Enum.ProductPurchaseDecision.NotProcessedYet
    end

    if info.ProductId == ProductID then
        player.leaderstats.Cash.Value = player.leaderstats.Cash.Value + 500
    end 

    return Enum.ProductPurchaseDecision.PurchaseGranted 

end

LocalScript inside of your TextButton.

local MarketplaceService = game:GetService("MarketplaceService")
local ProductID = 92041867

script.Parent.MouseButton1Down:connect(function()
    MarketplaceService:PromptProductPurchase(game.Players.LocalPlayer, ProductID)
end)

Edit: From your leaderboard script, it looks like the issue is that you're setting the value of cash to a separate value every 5 seconds. This overrides the value in the leaderboard. After processing your receipt you should be updating your dictionary rather than the leaderboard.

Here's a new basic leaderboard script. This script is both creating and updating the leaderboard and processing receipts.

local MarketplaceService = game:GetService("MarketplaceService")
local ProductID = 92041867

local playerLeaderstats = {}

game.Players.PlayerAdded:connect(function(player)

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

    local rank = Instance.new("StringValue", leaderstats)
    rank.Name = "Rank"
    rank.Value = player:GetRoleInGroup(3309505) 

    playerLeaderstats[player] = {}
    playerLeaderstats[player]["Cash"] = 100

    local money = Instance.new("IntValue", leaderstats)
    money.Name = "Cash"
    money.Value = playerLeaderstats[player]["Cash"]

    while wait(5) do 
        playerLeaderstats[player]["Cash"] = playerLeaderstats[player]["Cash"] + 1
        if player:FindFirstChild("leaderstats") ~= nil then
            player.leaderstats.Cash.Value = playerLeaderstats[player]["Cash"]
        end
    end

end)

MarketplaceService.ProcessReceipt = function(info)

    -- All of this code is a slighly modified version of the code on the wiki.
    local player = game:GetService("Players"):GetPlayerByUserId(info.PlayerId)
    if not player then
        return Enum.ProductPurchaseDecision.NotProcessedYet
    end

    if info.ProductId == ProductID then
        playerLeaderstats[player]["Cash"] = playerLeaderstats[player]["Cash"] + 500
        if player:FindFirstChild("leaderstats") ~= nil then
            player.leaderstats.Cash.Value = playerLeaderstats[player]["Cash"]
        end
    end 

    return Enum.ProductPurchaseDecision.PurchaseGranted 

end

Hi sweet, It is hard right now to know the solution because we don't know what your game is about. Please try using this and see if it works:

--PUT THIS IN A SCRIPT IN THE ServerScriptService

local MPS = game:GetService'MarketplaceService'

MPS.ProcessReceipt = function(info)
    local plr = game:GetService'Players':GetPlayerByUserId(info.PlayerId)
    if plr then
        local prodid = info.ProductId
        if prodid =  92041867  then
            if plr:FindFirstChild'leaderstats' and plr.leaderstats:FindFirstChild'Cash' then
                plr.leaderstats.Cash.Value = plr.leaderstats.Cash.Value + 500
                print('given')
            end
        end
    end
end

Now, put this code in a LocalScript in your Button

local buyButton = script.Parent
local productId = 92041867 --change to dev product id
local mps = game:GetService"MarketplaceService"

buyButton.MouseButton1Click:connect(function()
mps:PromptProductPurchase(game.Players.LocalPlayer, productId)
end)

Please test and if it works please click Accept Answer. If it doesn't, just tell me and I'll be happy to help :) Also, try to give me what the Developer Console (Local & Server) says if it doesnt work.

It requires parenthesis

local mps = game:GetService("MarketplaceService")