How do I make players get cash after a purchase of a Dev Product?

	local plr = game.Players.LocalPlayer

	local frame = script.Parent
	local robux = script.Parent.Parent.Parent.Parent.Parent.Robux
	local mps = game:GetService("MarketplaceService")
	local button1 = frame.b1000  local id1 = 36477699
	local button2 = frame.b5000  local id2 = 36477717
	local button3 = frame.b15000  local id3 = 36477751
	local button4 = frame.b85000  local id4 = 36477768
	
	button1.MouseButton1Down:connect(function()
		mps:PromptProductPurchase(plr, id1)
		
		mps.PromptPurchaseFinished:connect(function(plr, id1, purchased)
			if purchased == true then
				robux.Value = robux.Value + 1000
			end
		end)
	end)
	
	button2.MouseButton1Down:connect(function()
		mps:PromptProductPurchase(plr, id2)
		
		mps.PromptPurchaseFinished:connect(function(plr, id2, purchased)
			if purchased == true then
				robux.Value = robux.Value + 5000
			end
		end)
	end)
	
	button3.MouseButton1Down:connect(function()
		mps:PromptPurchase(plr, id3)
		
		mps.PromptProductPurchaseFinished:connect(function(plr, id3, purchased)
			if purchased == true then
				robux.Value = robux.Value + 15000
			end
		end)
	end)
	
	button4.MouseButton1Down:connect(function()
		mps:PromptProductPurchase(plr, id4)
		
		mps.PromptPurchaseFinished:connect(function(plr, id4, purchased)
			if purchased == true then
				robux.Value = robux.Value + 85000
			end
		end)
	end)

I tried in all of these the function '.PromptPurchaseFinished' and they don't work? I get no errors...help?