Need help in making a currency that saves?

There's already a useful leaderstats saver, put this into a regular script, and place it in Workspace:

function onPlayerEntered(player)
wait()-- Change to wait for player longer.
player:WaitForDataReady()
repeat wait() until player:FindFirstChild("leaderstats")
if player.DataReady then
if player:findFirstChild("leaderstats") then
local score = player.leaderstats:GetChildren()
for i = 1,#score do
local ScoreLoaded = player:LoadNumber(score[i].Name)
wait()
if ScoreLoaded ~= 0 then
score[i].Value = ScoreLoaded
end
end
end
end
end

function onPlayerLeaving(player)
if player:findFirstChild("leaderstats") then
local score = player.leaderstats:GetChildren()
for i = 1,#score do
player:SaveNumber(score[i].Name,score[i].Value)
end
end
end

game.Players.PlayerAdded:connect(onPlayerEntered)
game.Players.PlayerRemoving:connect(onPlayerLeaving)

To purchase currency with robux, you'll need to use Developer Products, if you want to allow players to buy the currency multiple times.