UpdateAsync isn't working - why?

So I'm trying to make a shop, and when a player buys an item the following code runs. Everything prints just fine, and even at the end it prints true. However, the async is not updated and still returns false. Why?

button = script.Parent
price = 738
player = button.Parent.Parent.Parent.Parent.Parent.Parent.Parent

data = game:GetService("DataStoreService"):GetDataStore("MWTestData1")

function inventory(old)
	if not old.arm then 
		print(player.Name.." hasn't bought the arm")
		old.arm = true
		game.ServerStorage.PlayerStats[player.Name].Stats.Points.Value = game.ServerStorage.PlayerStats[player.Name].Stats.Points.Value - price
		print(player.Name.." purchased "..button.Parent.Name.."!")
		
		pointskey = player.userId.."Points"
		data:IncrementAsync(pointskey, -price)
		points = game.ServerStorage.PlayerStats[player.Name].Stats.Points
		points.Value = data:GetAsync(player.userId.."Points")
		print(player.Name.."'s points: "..points.Value)
	else
		print("Arm already purchased")
	end
	print(old..": "..old.arm)
	return old
end

function buyItem()
	print(player.Name.." is attempting to buy "..button.Parent.Name)
	if game.ServerStorage.PlayerStats[player.Name].Stats.Points.Value >= price then
		print(player.Name.." has enough points")
		data:UpdateAsync(player.userId.."Inventory",
			inventory)
	else
		print(player.Name.." doesn't have enough points")
	end
end


button.MouseButton1Click:connect(buyItem)

Thanks! --MightyWanderer

UpdateAsync's second argument is a function that returns the new value.

Here's the example the Roblox wiki uses to explain it:

function timesTwo(input)
    return input*2
end
 
ds = game:GetService("DataStoreService"):GetGlobalDataStore()
ds:UpdateAsync("mykey", timesTwo) -- This will change the value of "mykey" to the output value of the 
                                  -- timesTwo(value) function, with '''''value''''' as the current value for "mykey".

The upside of UpdateAsync is that there is no risk of an out-of-date value being used.