@Michael1279, either the host made a mistake when pricing the offer, or they are just trying to make more money off of you.
Either way, it is not your resposibility to solve the problem for them. And you should not send them any money outside AirBnB. You definitely should not give them a $300 check as a "security deposit".
If the host made a mistake, they need to deal with AirBnB. They made an offer to you; you accepted it. You are under no obligation to accept any changes. If the price is unacceptable to the host, the host needs to cancel, not you.
If the host is just trying to make more money off of you, then you are *definitely* not obligated to accept any changes.
Is the host at least communicating with you through the AirBnB message system? (You definitely want there to be an official record of what they are saying to you.) Everything you mention the host is trying is sketchy and outside the official AirBnB rules.
And don't cancel.