Can you forgive someone who hit you? Or does someone have to pay for his violence against you? Does his hitting you automatically give you the right to hit him back? Of course God can freely forgive us. There's no such thing as sin, therefore there is nothing to have to pay back to God. that's just church Kool-Aid and you have taken a BIG swig of it.
Personal forgiveness is different from atoning for someone's sin. That person doesn't have to pay for their wrongdoings because it's not my place to judge them. God can't freely forgive us for the same reason that a judge can't freely forgive a person who committed a crime. Why do you think there's no such thing as sin? Don't we all do things that we know we aren't supposed to do?