Anything that lets someone do something with your property is an asset to that person, and a liability to you. So both the lease and the option are assets to the tenant, and liabilities to the estate. The only assets that the estate has is the property and the $25,000 payment (either as cash or accounts receivable, as the case may be).
The estate is pretty much guaranteed to end up with not much more than $50k; if the property ends up being worth more than that (plus transaction costs), the tenant would be foolish to not buy, and if it ends up worth less (which would require either a massive overassessment and/or something catastrophic in the housing market), then the estate will be left with a worth-less-than-50k property.
Since you're getting $25k of that at the end of the lease, if you want that in present value you'll have to do time discounting, and if the other $25k is being received over the course of the lease, then you'll have even more (and more complicated) time discounting.
The simplest accounting would be to take the time discounted $50k as current assets, ignore the value of the property itself, since you no longer have any claim to it (during the lease, the tenant is entitled to its use, and at the end of the lease, it almost certainly will be bought), and if the house decreases below $50k, treat that as an expense. A slightly more complicated version of this would be to estimate the expected value of this loss (integral of (50k-x)(p(x)) from 0 to 50k where p(x) is the probability of the house being worth x dollars) and add that to the $50k (this will be a negative number, so adding it will decrease the amount).
If you want to be really fancy, you can enter the property as an asset, set some value for it (the assessment would be one candidate, but as Pete B. said, assessment is not the same as market value), then value the option to buy. Figuring out the market value of a property is nontrivial, and figuring out the market value of an option on a property is even more difficult; it's not like there's a market of people trading options on your property. On the other hand, with it being so far in the money, its value depends almost entirely on the value of the property. As the property is an asset, and the option is a liability, any value over 50k cancels out (the property being worth more means the option is also worth more, making the effect on your total value negligible). So while this avenue is arguably more complete from an accounting point of view, it's adding a bunch of complexity and uncertainty that isn't really worth much from a practical point of view.