Bug 28994: Fix logical errors with amount vs amoutoutstanding

Bug 28994: Fix logical errors with amount vs amoutoutstanding

The writeoff debts script contained an error whereby we could writeoff
more than the current outstanding debt of a charge.  This patch corrects
that mistake by passing amountoutstanding in place of amount in the
writeoff and offset creation lines.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>