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