No Refunds?

We've been having a friendly argument lately with a friend of ours that believes in the Christian principle of "once saved, always saved". We are all Christians but we disagree on this point.

Here is the first verse we found that seems to disagree with it:

Hebrews 10:26-27 (Holman Christian Standard Bible)
26 For if we deliberately sin after receiving the knowledge of the truth, there no longer remains a sacrifice for sins, 27 but a terrifying expectation of judgment, and the fury of a fire about to consume the adversaries.

Our Friends response to this was that Jesus himself never said salvation could be lost so this verse couldn't be talking about loss of salvation or the Bible would be contradicting itself.

So I focused my efforts on what Jesus had to say on the subject and found this:

"For if ye forgive men their trespasses, your heavenly Father will also forgive you: But if ye forgive not men their trespasses, neither will your Father forgive your trespasses." ( Matthew 6:14-15)

This verse shows that refusing to forgive the sins of your brothers will cause your sins to not be forgiven and this offense could occur after one is saved. If your sins aren't forgiven, then how do you get into Heaven? Their argument that Christ's death on the cross covers all sins past and future once you accept him doesn't seem to hold up here. Jesus is saying that those sins are not forgiven, so how are they still covered by grace?

If this type of sin could cause you to lose your salvation then why couldn't others?

If your response to that is that the lack of forgiveness is just a cause for less blessing in heaven and not a loss of salvation then I think that that would contradict this verse (spoken by Jesus)

And if your eye causes you to sin, pluck it out. It is better for you to enter the kingdom of God with one eye than to have two eyes and be thrown into hell,
Mark 9:46-48

This verse shows that sin sends people to hell, and he doesn't say "only the sin of the unsaved", so I must assume he means everyone, since only what Jesus literally says is admissible based on the first comment from our friends.

Our friends also disagree with that saying that the second verse is taken out of context and that Jesus was in fact talking about the unsaved. In all fairness I can't see that as I reread the passage. It seems to me that I took the literal meaning of the verse for use in my arguments.

I guess I just don't understand and in the interest of our friendship we agreed to disagree, but in conclusion I will just say this. If God gave us the free will to choose him, why then would he not also allow us to change our mind and return to sin? Is free will only free as long as we haven't already chosen to believe?