"钱袋口上的绳子" can be translated into English as "the string at the mouth of the money bag" or more naturally as "the drawstring of the money pouch".
"String at the mouth of the money bag" directly describes the location and function of the rope (string) on the money bag.
"Drawstring of the money pouch" is a more idiomatic and concise expression, where "drawstring" refers to a cord or string used to tighten or close an opening (like a bag or pouch), and "money pouch" is a common term for a small bag used to carry money.