What is the ruling on relinquishing my claim to my grandfather's inheritance (an apartment and a shop) that he bequeathed to me, after the forgery of the amicable division document by my uncle has been proven? And what must I do according to Islamic law?

The resolution of disputes takes place in Sharia courts because they hear from all parties and issue a judgment based on that. As for the Mufti, he only hears from one party.

The will is valid for you if it is proven that your grandfather bequeathed that will to you legally, and the heirs do not have the right to deprive you of it. If the bequeathed items (the apartment and the shop) exceed one-third of your grandfather's estate, then you are only entitled to the amount of one-third, because a bequest to a non-heir is limited to one-third, and anything in excess depends on the heirs' consent.

You are permitted to take the case to court to claim your right, but you are not allowed to lie or swear falsely before the judge.

Repentance is valid for every sin. So, whoever swore falsely must feel remorse and resolve not to return to lying. If their oath resulted in the loss of someone's right, then it is obligatory to restore it to them or guarantee it.