1. Create invoice $120
2. Create payment $120
3. Particually allocate invoice to payment for $100, another $20 writeoff from invoice by Charge.
4. Posting of Allocation: $20 Charge Expense in Credit (see picture).
5. Reverse Allocation. Posting of reveral allocation is incorrect: $20 Charge Expense in Debit, but must be in Credit with minus. (see picture)
Testing on test.idempiere.org (v20180105-1454)
still reproducible on 7.1
the problem looks like to be caused by this