Thanks for your enquiry.
I am afraid to say that if the Will stipulates that your Uncle's ex lodger has the right to purchase the property at 35K, and he agrees to this, then you aren't entitled to put any such condition in the Sale agreement.
The Will gives him a right to take up this offer and if he buys, then he is then free to do with the property as he so wishes.
I am sorry this is not the answer you were looking for but it sets out the legal position.
If I have answered your question, I would be grateful if you could leave positive feedback.