What is the ruling on saying: "My soul be sacrificed for a grave you inhabit, in which there is chastity, generosity, and benevolence," and is this considered exaggeration?

Ransoming the grave is not considered exaggeration (ghuluww). If the speaker intends to ransom the Prophet's person (peace be upon him) and expresses this by metaphorically saying "ransoming the grave," then there is no problem with it. This is because the sanctity of the Prophet (peace be upon him) in death is like his sanctity in life. Or, if he intends to protect his grave (peace be upon him) from grave robbers and aggressors, then there is no harm in it, as graves have sanctity, and the grave of the best of creation is more deserving.