How do i fix this dev products problem?

I was going to make a comment on this, then I saw way to many incorrect things. First, handling the purchase and the purchasing prompt MUST be in two different things. In the player: (any storage, PlayerGui, StarterPack, works. But if this is for a gui in a player, must be in the gui, in the PlayerGui for effiency.) simple, in a local script, is where you prompt the purchase.

local buyButton = script.Parent
local productId = 24735683
local player = game.Players.LocalPlayer -- script.Parent.Parent.Parent etc. is just so inefficient.
local MarketplaceService = game:GetService("MarketplaceService")

buyButton.MouseButton1Click:connect(function()
    MarketplaceService:PromptProductPurchase(player, productId)
end)

Next, this is where you handle the purchase, if it was made. No need for iteration.

local productId = 24735683
local MarketplaceService = game:GetService("MarketplaceService")

MarketplaceService.ProcessReceipt = function(receiptInfo) 
player = game.Players:GetPlayerByUserId(receiptInfo.PlayerId)
        if player.userId == receiptInfo.PlayerId then
     if receiptInfo.ProductId == productId then
    player.PlayerGui.Money.MonV.Value = player.PlayerGui.Money.MonV.Value + 500
            end
        end
    end 

    return Enum.ProductPurchaseDecision.PurchaseGranted     
end

You could also save it using DataStores, but thats not what you asked for, so i wont go there. You were very close, your program was just jumbled and all in one. You cannot handle both purchase and prompt in one. It just doesn't work right, personally, and then the efficency errors. But you main problem was the "for" loop I believe.