Can a landlord cancel the lease before tenant moves in

Depends on what commitments have been made. If you both signed a lease, that lease is binding unless the tenant lied on their apllication.

Yes we did signed the lease.but I don't if she lied on her application,I am about to modicate my loan and I heard that if it is rented out I won't be qualified for this.Thats why I want to cancel lease before she moves in.

Sorry, if there is a signed lease you are obligated if there is no reason according to the lease to void it.

Just as you can hold them liable at this point they can hold.
Or you can offer to buy the lease out.

What is to buy lease out.What shold I pay?

Generally about 2 months rent.