To whom is this "debt" owed that it must be paid?
I could loan $5 or $5000 and decide to cancel the debt without someone ELSE needing to pay it ... so why can God not simply "forgive" as He asks us to? [We do not demand another pay the "pound of flesh" that is owed us for the wrong we suffer.]
[PS. Just to be clear, I liked your response. I just thought this point could use a little more discussion and clarification.]
That is the problem with taking the way a biblical idea was formed in the culture in which it was written, and making an analogy from our cultural understandings.
In the OT law there was a law of the kinsman redeemer that more closely relates to this concept of debt than our money system of debt and repayment. The kinsman redeemer was a male relative who was responsible to help a relative in need or danger. This person delivers, rescues, and sometimes redeems property. The kinsman redeemer laws are found in Lev 25:25-34.
One of Jesus' titles is Redeemer. He delivers and rescues. In the kinsman redeemer law there was also provision for a relative to by back the property of one who had to sell it because of his poverty or sold himself into servitude to another because of his poverty.
The "debt" we owe is to God. We owe Him as creatures He created in His image and likeness, perfect righteousness and full loyalty and obedience to only Him. Our ability to do this has been utterly destroyed by the transgression of Adam. We became creatures that sin. And God is holy, holy, holy.
So the debt is not one that in any way relates to monetary comparisons. It is much bigger than that and there is more at stake than just a simple forgiveness of our sins. Put one way, the devil is the slaveholder, and our sins are the chains that bind us to him, and keep us from God. By God's decree and wisdom, the penalty for sin is death. We are doomed.
God's justice in meeting His own decrees against sin must be met. Sin can't just be forgiven, it must be destroyed. In comes the Redeemer, perfectly righteous, no spot of blemish on Him, who substitutes Himself in the place of the prisoner and meets death for them. Walks right into the open maw of death taking the sins of His people with Him, and conquers sin and death, by rising from the dead for it cannot hold a righteous one. That is the debt satisfied by our Kinsman Redeemer.