What is the Old Testament Biblical basis to the idea that only God can forgive sins? Mark 2:7 and Luke 5:21 have scribes and Pharisees claiming that only God can forgive sins.

&quot;Why does this man speak like this? He is blaspheming! Who can forgive
sins but God alone?&quot;

and

&quot;But the scribes and Pharisees began thinking to themselves, “Who is
this man who speaks blasphemy? Who can forgive sins but God alone?”&quot;

What is the Old Testament Biblical basis to the claim that God alone can forgive sins, and not one who is delegated with authority by God?
Under the Jewish theocracy, wilful disregard of the positive, or wilful infraction of the negative, commands of God as proclaimed by Moses and interpreted by the Rabbis; it thus includes crimes against God and crimes against society or an individual member thereof.
…
The earliest Biblical conception of what constituted sin is illustrated by the story of Adam's punishment, which was due to his failure to obey the divine will and his revolt against the divine government.
The catastrophe of the Flood was a punishment for man's demoralization and corruption, his violence and immorality (see Gen. vi. 11, 12).
The builders of the Tower of Babel revolted against divine government, and were dispersed (see Gen. xi. 1-9).
Sodom and Gomorrah were destroyed for their heinous crimes: &quot;The men of Sodom were wicked and sinners before the Lord exceedingly&quot; (Gen. xiii. 13): they were &quot;wicked&quot; in civil matters, &quot;sinners&quot; in blasphemy &quot;exceedingly,&quot; with full appreciation of the enormity of their sins (Sanh. 109a).
The Egyptians were punished for the sin of enslaving the Israelites, and for not heeding the command of God to release them.
The most serious sin of the Israelites was the worship of the golden calf, contrary to God's commandments delivered from Sinai.
Korah rebelled against the authority of Moses, and of the Levites, priests by the choice of God.
The Canaanites practised incest and immorality: &quot;For they committed all these things, and therefore I abhorred them&quot; (Lev. xx. 23); &quot;But for the wickedness of these nations the Lord thy God doth drive them out from before thee&quot; (Dout. ix. 5).
…
Jewish theology does not admit that there is an unpardonable sin. The Mishnah says that sins are expiated (1) by sacrifice, (2) by repentance at death or on Yom Kippur, (3) in the case of the lighter transgressions of the positive or negative precepts, by repentance at any time.
— SIN - JewishEncyclopedia.com

The dominant theme here is that sin is willful disobedience of God's commandments.
As 1 John 3 explains it in the New Testament:

Whosoever committeth sin transgresseth also the law: for sin is the transgression of the law.

Sin is disrespecting God.
The idea that anyone other than God himself could forgive direct attacks against him just doesn't make sense.
Suppose someone did something awful against you.
Would it make sense for me to forgive them?
Would you ever consider delegating someone else to offer forgiveness every time someone hurts you?
Or consider the reverse analogy.
Would it ever make sense to delegate your ability to love someone to a third person?