This is as a local script

-- Buy Product Script --

--GetService--

local Players = game:GetService('Players').LocalPlayer

local plr = game.PlayersLocalPlayer
local leaderboard  = plr:WaitForChild("leaderstats")
local Cash = leaderboard:WaitForChild("Cash")

--local plr = Players.LocalPlayer -- LocalPlayer can only be used in a LocalScript 

--VARS--

local ItemId = 328586978 --Put DevProduct Id Here

--Buy Screen Pops Up--
game.Workspace.AllowThirdPartySales = true
script.Parent.MouseButton1Click:Connect(function()
	
	--Prompt Purchase Screen--
	Cash:PromptProductPurchase(plr, ItemId)
	
end)

How do I make it where if it went through, than it will add cash?

I added this under --Prompt Purchase Screen---

Cash.Value + 250
Anonymous