Can anyone help me? Why doesn't the value change?

Can anyone help me and how to check if it's a number? ServerScript in ServerScriptService:

script.Parent.Event.Banken.OnServerEvent:Connect(function(player, what, val)
    if what == 5 then
        player.stats.Geld.Value = player.stats.Geld.Value-val
        player.stats.Bank.Value = player.stats.Bank.Value+val
    elseif what == "afhaal" then
        player.stats.Geld.Value = player.stats.Geld.Value+val
        player.stats.Bank.Value = player.stats.Bank.Value-val
    end
end)

Local script in a GUI:

local storten = script.Parent.Storten
local afhalen = script.Parent.Afhalen
local geld = script.Parent.geld
local updatetext = script.Parent.Wat
local banken = game.ReplicatedStorage.Events.Banken

function stort()
    local value = tonumber(script.Parent.geld.Text)
    local what = 5
    banken:FireServer(what, value)
    updatetext.Text = "Je hebt ".. value .." gestort."
    updatetext.Visible = true
    wait(5)
    updatetext.Visible = false
end

function afhaal()
    local value = tonumber(script.Parent.geld.Text)
    local what = "afhaal"
    banken:FireServer(what, value)
    updatetext.Text = "Je hebt ".. value .." afgehaalt."
    updatetext.Visible = true
    wait(5)
    updatetext.Visible = false
end

script.Parent.Close.MouseButton1Click:Connect(function()
    script.Parent.Parent:Destroy()
end)



storten.MouseButton1Click:Connect(stort)
afhalen.MouseButton1Click:Connect(afhaal)