As a native speaker I can say that there is no metaphorical meaning I know or could think of. "Haft" in context of prison is a legal term and only applies if a judge sentences someone to a kind of incarceration. This only made sence if you were currently in prison and it would never appear in a letter regarding of rental agreements - only in letters from a court, judge or possibly lawyer.
Like almost every word, "Haft" can have different meanings depending on the context. It can mean "imprisonment", "liability" or "adhesion". It can't be the legal term that means "incarceration" (since you can't be released from prison if you are not in prison) and it is obviously not "adhesion" so the only possible meaning is "liability" even though that is i.m.o. the least common one and is more likely to be translated as "Haftung" (Though it is correct to use "liability" for "Haft" and vice versa). It is the only meaning that makes any sence here.
You propably asked to terminate the rental agreement and they agreed and clarified the date when the liabilities end.
No metaphore, just uncommon wording and the mutual agreement not to continue the rental agreement after 2019-05-31.