If you do not agree with the proposed sale you can certainly challenge this by petitioning the probate court. Before doing so, I would get at least two other appraisals on the property, showing that the value is way off. If the sales price is in line with the appraised value, then you need something to counter that, in order for the judges to give you consideration.
Obviously, a traditional sale would be more beneficial to the heirs because they would get paid sooner and not be in a position of financing the buyer. But in these difficult economic times, having a sale at all is sometimes worth the added hassle. One alternative is that you or another one of the heirs might offer to purchase the property. If you can secure a traditional loan, that would be much more attractive for the court.
It would be helpful to you to have a lawyer present this, but it is not absolutely essential. You DO need to be prepared to document your position and to make sure that this does not come across as just another disgruntled heir story.
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FYI, the person administering the estate is the "personal representative". The probate court is always available to sort out disputes like this, as Mr. Frederick points out.
You will have to decide whether it is worth going to court or not. If you do go, either get a lawyer or get as much information as possible that you can present to the court.
I am licensed to practice law in Michigan and Virginia and regularly handle cases of this sort. You should not rely on this answer. You should consult a lawyer so you can tell the lawyer the entire situation and get legal advice that is precisely tailored to your case.
I agree with Mr. Frederick and Conways comments- however I would add: Since you are referring to the person as the "excutor" and not the personal representative it is possible they have not opened the estate and if that is true cannot convey good title regardless of to whom and for what price. Land Contracts can be written agrements between parties that if the parties don't go through attorneys and/or title companies they create a mess to be dealt with in the future.
By that I mean - if there is no probate case and the written contract is executed between the "executor" and other heir that wants to buy and payments are made on that contract it gets even more complicated than just going to probate court and objecting to the sale. If the "executor" has not officially been appointed by the probate court in the county where the person who died resided- you may want to insist it be opened cause its a real estate title mess if payments are made and the deal is signed without proper authority of the court.