Rabbinical theology suggest you forgive someone 3x if they do wrong. Peter thought he might please Jesus by stepping it up and forgiving 7x. Jesus says none of those are sufficient, but 70x7 which is 490x you should forgive. That's not the point though.
490 is merely an example. What Jesus meant was you should forgive unumerable times. Jesus points out if you could forgive that much that it might just become part of you; therefore becoming more like Him.
The man in this story was forgiven his debt of what would amount to in our day as millions of dollars. He turned around and pointed out a man in debt to him demanding he be paid back immidiately.
If Jesus made you pay back your debt you'd be dead. He has forgiven us of all our our debt, why can't we forgive others?