This script uses "amount" to write off debts, when it should be using "amountoutstanding":

                # A 'writeoff' is a 'credit'
                my $writeoff = Koha::Account::Line->new(
                    {
                        date              => \'NOW()',
                        amount            => 0 - $line->amount,   <== HERE
                        credit_type_code  => 'WRITEOFF',
                        status            => 'ADDED',
                        amountoutstanding => 0 - $line->amount,   <== HERE
                        manager_id        => undef,
                        borrowernumber    => $line->borrowernumber,
                        interface         => 'intranet',
                        branchcode        => undef,
                    }
                )->store();

                my $writeoff_offset = Koha::Account::Offset->new(
                    {
                        credit_id => $writeoff->accountlines_id,
                        type      => 'WRITEOFF',
                        amount    => $line->amount   <== HERE
                    }
                )->store();

Patch coming.