Response validation can be a very handy way to ensure your users are entering their answer in a format you are expecting. It's a relatively hidden option but can be well purposed to set up a question allowing users to specify a payment amount using a Short Answer question.
Adding a Response Validation question
Adding response validation ensures your user puts in the proper number format, and you can also set a minimum and maximum if needed.
Make sure you add a $ or money identifier to the end of the title of your short answer question. This way, we know the number in the response will be added to the Payable invoice.
Money identifier in question title