How do you check if text is a number?

You can use the tonumber() function to convert a string to a number. If the string has no number representation then the function returns nil. This function will also return nil if the string has something other than a number; for example, tonumber("1234f") will return nil because it has a letter in it.

local TextBox = script.Parent

TextBox.FocusLost:Connect(function()

    local Amount = TextBox.Text

    if tonumber(Amount) then
        -- Code
    else
        TextBox.Text = ""
        TextBox.PlaceholderText = "Not a valid amount of cash!"
    end
end)

Documentation for Converting Strings