Terry, I cant yet answer your question about the differences between Mark and Luke but I stumbled about this one, where Jesus used the "DEBTS" illustration too.
In
the „Parable of the wicked slave“ God's standard and willingness
to forgive is addressed.
Gods behave is an example for man. In jewish faith mercy, benevolence and readiness to forgive are ordered to man. Jesus illustrated the personal failures or transgressions as „debts“.
What is remarkable about God in the parable is that the
„king“ released the „wicked slave“ or "unmerciful obligee"
from an awesome unimaginable high sum of debts (10000 talents* ~ USD 40 Mio. )
compared to the insignificant 100 denarii. (~ USD 70)
One matter is clear Gods standard is forgiveness.
The "wicked slave" does not forgive, comparable to a wicked slave class of people that we know well, the wicked slave class perhaps really exists.
Read the text as in NASB
https://www.biblegateway.com/passage/?search=matthew+18%2C23&version=NASB
“For this reason the kingdom of heaven may be compared to a king who wished to settle accounts with his slaves. When he had begun to settle them, one who owed him ten thousand talents was brought to him. But since he did not have the means to repay, his lord commanded him to be sold, along with his wife and children and all that he had, and repayment to be made. So the slave fell to the ground and prostrated himself before him, saying, ‘Have patience with me and I will repay you everything.’
And the lord of that slave felt compassion and released him and forgave him the debt. But that slave went out and found one of his fellow slaves who owed him a hundred denarii; and he seized him and began to choke him, saying, ‘Pay back what you owe. So his fellow slave fell to the ground and began to plead with him, saying, ‘Have patience with me and I will repay you.’But he was unwilling and went and threw him in prison until he should pay back what was owed.
So when his fellow slaves saw what had happened, they were deeply grieved and came and reported to their lord all that had happened. Then summoning him, his lord *said to him, ‘You wicked slave, I forgave you all that debt because you pleaded with me. Should you not also have had mercy on your fellow slave, in the same way that I had mercy on you?’ And his lord, moved with anger, handed him over to the torturers until he should repay all that was owed him. My heavenly Father will also do the same to you, if each of you does not forgive his brother from your heart.”