Is it possible to waive a share of an inheritance on condition of receiving one third of the amount when the inheritance is sold later, and how can the right be guaranteed in this case?

The jurists differed regarding the validity of a gift when it is accompanied by a condition that contradicts its nature. The majority of them hold that the condition is void but the gift is valid. However, the Malikis, the Shafi'is (within their school of thought), and the Hanbalis (according to one view) hold that both the contract and the condition are void. As for relinquishment that means gifting the usufruct (manfa'ah) without the ownership (raqabah), it is permissible in Sharia and is considered a loan ('ariyah).