Okay, let me rephrase the question.
If God has forgiven someone, declaring that he will not hold his or her sins against them, then why do you say that the single sin mentioned in Romans 4:23 nullifies God's promise?
The same God that promises He forgets
sin also forgets
righteousness.
Ezekiel 18
24But if a righteous man turns from his righteousness and practices iniquity, committing the same abominations as the wicked, will he live?
None of the righteous acts he did will be remembered. Because of the unfaithfulness and sin he has committed, he will die.
In the New Covenant, faith is counted as righteousness, so the righteousness God "forgets" is that the person had faith--their name is blotted out of the Book of Life, and the debt that had been forgiven and forgotten returns before God.
Matthew 18
23“For this reason the kingdom of heaven may be compared to a king who wished to settle accounts with his slaves.
24“When he had begun to settle
them, one who owed him ten thousand talents was brought to him.
25“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.
26“So the slave fell
to the ground and prostrated himself before him, saying, ‘Have patience with me and I will repay you everything.’
27“And the lord of that slave felt compassion and released him and forgave him the debt.
28“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.’
29“So his fellow slave fell
to the ground and
began to plead with him, saying, ‘Have patience with me and I will repay you.’
30“But he was unwilling and went and threw him in prison until he should pay back what was owed.
31“So when his fellow slaves saw what had happened, they were deeply grieved and came and reported to their lord all that had happened.
32“Then summoning him, his lord said to him, ‘You wicked slave, I forgave you all that debt because you pleaded with me.
33‘Should you not also have had mercy on your fellow slave, in the same way that I had mercy on you?’
34“And his lord, moved with anger, handed him over to the torturers until he should repay all that was owed him.
35“My heavenly Father will also do the same to you, if each of you does not forgive his brother from your heart.”
Therefore, we see, again, that being "in Christ" is not a static state, but, unfortunately, can, and does, fluctuate; this is the only satisfactory explanation for how "there is therefore now no condemnation for those in Christ", yet the "true believer" who sins "is condemned" (he's not remaining in Christ, because that is by obeying commands, and, being a sinner, he is, ipso facto, breaking commands).