I’m not aware of any Tiller tool that would do that but do have a thought on one way to handle the situation you might want to consider.
You could use a Tag such as AmazonLoan when you categorize the original charge transaction to the appropriate expense category. Then when payments are subsequently posted, you could Tag them to the same AmazonLoan and categorize it as a Transfer.
You can then run a Tags Report and see the remaining balance of your loan whenever you want.
Let me know if that looks like it would work for you,
Fred
P.S. Here’s a link to the Tags Report documentation: