It seems partial payments and reversing fines don't work well together.
1) Create a manual invoice of 2.00 (Sundry), total due is 2.00.
2) Select the fine, click on 'Pay Selected' and pay off 1.00. Total due is 1.00.
3) Writeoff the additional 1.00 by clicking on 'Writeoff' next to the fine. Total due is now 0.00
4) Reverse the 1.00 payment. Total due will be 2.00 now, instead of 1.00.
This works correctly for me on current master. Closing bug.