Contradiction with James 5:16
While Luke 5:23 implies forgiveness is given directly, James 5:16 emphasizes confession to one another for healing and forgiveness.
James 5:16: Confess [your] faults one to another, and pray one for another, that ye may be healed. The effectual fervent prayer of a righteous man availeth much.
Contradiction with Exodus 34:7
Luke 5:23 implies immediate forgiveness, but Exodus 34:7 states that God does not clear the guilty, suggesting a complex process of forgiveness.
Exodus 34:7: Keeping mercy for thousands, forgiving iniquity and transgression and sin, and that will by no means clear [the guilty]; visiting the iniquity of the fathers upon the children, and upon the children's children, unto the third and to the fourth [generation].
Contradiction with Hebrews 10:18
Luke 5:23 suggests continuing need for forgiveness, whereas Hebrews 10:18 notes that where there is forgiveness, no more offering for sin is needed.
Hebrews 10:18: Now where remission of these [is, there is] no more offering for sin.
Contradiction with Matthew 6:14-15
Luke 5:23 poses a direct question of forgiving sins versus healing, but Matthew 6:14-15 conditions forgiveness on forgiving others.
Matthew 6:14-15: For if ye forgive men their trespasses, your heavenly Father will also forgive you:
Contradiction with Jeremiah 31:34
Luke 5:23 implies an ongoing role of Jesus in forgiveness, whereas Jeremiah 31:34 speaks of a future where God Himself forgives and remembers sins no more.
Jeremiah 31:34: And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more.