I have been staying in a complex more than 5 years(from 2006 Mar onwards) and lease was getting extended every year after 12 months. In the first lease they have clearly mentioned the lease breakage clause as 'We agree to let you vacate your apartment and terminate your lease prior to the termination date subject to the following provisions;
1-You must provide written notice of your intent to vacate at least 60 days before termination date
2-you will be assessed a lease buy-out fee based upon the remaining percentage of your existing lease term
25% or less of existing lease term - one month rent as feee
50% or less of existing lease term - two month rent as fee
75% or less of existing lease term -three month rent as fee

However later when we renewed in 2010-2011 , the above clause was removed - However there is another clause mentioned under Security Deposit' as "You has given us total deposit of $855.Upon termination of tenancy, we may keep all or part of the deposit if you do any of the following
1-You break your lease by vacating your apartment prior to the termination date of the lease and we are unable to re-rent the apartment of any portion of unexpired term of the lease.

Is this means I need to pay max $855 in case if I break the lease ? Now Leasing office asking me to pay $1800/- which is not mentioned anywhere in the rental agreement. Please help me to understand. Does it not needed to mention the lease breakage clause in the rental agreement?
Joe

Every lease can be completely rewritten and terms changed after expire... they are only valid for the duration of that specific lease.

YOU have to pay what is actually spelled out in your lease. Assuming of course those fees are not considered excessive under state law.

Technically and legally... that can not only keep that fee, they can additionally hold you liable for rent for the full term of the lease, assuming they have actively marketed the unit and attempted to find a new renter and were unable. As well as any damages beyond customary wear and tear.

But the better places are more lenient if you have been both a good tenant and have been there for a long time.

Some of this depends on your state.
Some states are very specific about the limits on what a security deposit can be taken for (damage, generally), regardless of what it says in a lease.
As for the 1800 they expect, the owner can actually now charge you the entire rest of the year due on the lease, unless they rent it out. And again depending on the state, they may be able to charge what they want as a fee for finding the new tenant, or your state may set a limit.
I am guessing that 1800 is 2 months rent? That might be considered reasonable.
If you are in a college city, the apartment may sit vacant until September. Consider yourself lucky.
One thing you can do is find a new tenant who passes their review.

I have only 2 month left out to complete lease

How is that, if your lease ran March to March?

...
Is this means I need to pay max $855 in case if I break the lease ? Now Leasing office asking me to pay $1800/- which is not mentioned anywhere in the rental agreement. Please help me to understand. Does it not needed to mention the lease breakage clause in the rental agreement?
Joe

No. It means that they can use the deposit to compensate them should you terminate early. They can require you to pay more than that.

But since the "lease breakage clause" was not included in the current lease, they have the standard right to insist that you pay for the entire term of the lease, unless they are able to re-let the premises for some portion of the balance of your lease term. In other words, the law requires that you pay for early temination of a lease, whether there is such language in the lease.