How to use Developer Products?

I don't want there purchase to save though, so they can re-buy it.

When I press the button it gives them the cash without even buying it. I want to make sure they press that buy button and THEN give them the cash.

What do I do with this?

  MarketplaceService.ProcessReceipt = function(receiptInfo)
   return Enum.ProductPurchaseDecision.PurchaseGranted  
  end    

wait(1)
local player = game.Players.LocalPlayer
local button = script.Parent
local productId = button.ID.Value
local label = script.Parent.Parent.Parent.Parent.Parent.Currency.CurrencyFrame.Money
local value = label.Amount
local cash = button.Cash

--This is where I am having problems.
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button.MouseButton1Click:connect(function()


    Game:GetService("MarketplaceService"):PromptProductPurchase(player, productId)
    MarketplaceService.ProcessReceipt = function(receiptInfo)
   return Enum.ProductPurchaseDecision.PurchaseGranted  
    end    
   value.Value = value.Value + cash.Value
end)
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