Answer:
No, the uncle's action to the buyer was not effective.
Step-by-step explanation:
His uncle executed a deed of guarantee that gave the nephew a plot of land and did not undo the action. So the nephew is the natural heir, as the statute of the jurisdiction in which the land is located provides that no transfer or mortgage of real estate will be valid against subsequent buyers for value and without notice whose transfer is first registered, from according to the law.