Nebuchadnezzar's sins, conceived of as debts, have risen to such a level that his creditor, God, is about to demand repayment in the formof punishment: He is calling in the bond he holds over this unfortunate debtor.
The Lord's Prayer in Luke contains the petition «Forgive us our sins (11:4), and it is perhaps unfortunate that we do not commonly use this form instead of «debts» or «trespasses.»