What is the value of the fees due to the petitioner for his effort, time, and money in recovering the building confiscated by the state for thirty years, with the heirs' approval to deduct it from other than his share of the inheritance?

If the heirs appointed you as their representative in a dispute and you spent money with the intention of being reimbursed, then you may claim what you have spent. The compensation can be estimated either by calculating the fair market value of your services along with what you spent, or by agreement with the adult, sane heirs on a specific compensation. Since they have agreed to your receiving compensation, the fees should be estimated and a specific compensation agreed upon, and whatever is agreed upon is permissible.