The tenant has a lease and there isn't any place I know of where you
can terminate a lease because the property is being sold, unless that
is spelled out in the lease. It doesn't matter if the landlord intends
to now use it for his own use. A lease is a binding legal contract.
As far as the security deposit, I would make sure where that is, how
much it is, and that what has been done conforms to local law. Some
places have strict laws requiring that it be placed in an interest
earning account, promptly returned, etc. And if it's not, the tenant
may be able to recover more than the security deposit. I would have
the security deposit spelled out in the purchase contract and that it
is being turned over to your control or given to you, etc.
A key consideration here is the tenent. I would try to find out as
much as possible. Since they are paying a substantial rent, you are
less likely to have problems, but that is still possible. Giving the
tenant an incentive to leave, as suggested, could be a good idea. You
could offer a bonus equal to two months rent if the tenant leaves
within say 90 days.
As to how much to discount the offer price, that's almost impossible to
seperate from all the other factors involved. Like how much you like
the place, how easy it would be to find another place, how hot/cold the
current market is, etc. From a hard numbers standpoint, you could
calculate what the carrying cost of the place is for interest, taxes,
maintenance, etc for the remainder of the lease. Factor in that the
loss is tax deductible, which helps. If you come up with say $10K,
then that is a general guide. In any case, unless it's a hot market
and you want it bad, I'd tend to make the offer on the low side, as you
can always come up.