Indiana does not have a statute addressing a landlord's duty to mitigate. Indiana courts, instead, apply a common law doctrine as explained by the Indiana Court of Appeals in Nylen v. Park Doral apartments:
https://scholar.google.com/scholar_c...en&as_sdt=4,15The doctrine of mitigation of damages creates an obligation on the part of the landlord to use such diligence as would be exercised by a reasonably prudent man under similar circumstances to re-let the premises, if possible, in order to mitigate damages resulting from the tenant's breach of lease. State v. Boyle (1976), 168 Ind. App. 643, 646, 344 N.E.2d 302, 304. The obligation exists even if there is no mandatory re-letting clause in the lease. See Hirsch et al. v. Merchants Nat'l Bk. (1975), 166 Ind. App. 497, 336 N.E.2d 833. Further, courts have recognized and enforced the doctrine of mitigation of damages while at the same time sustaining savings clauses.
Id.; see also Grueninger, supra, 413 N.E.2d 1034.
The prior cases cited say pretty much the same thing and decide whether or not the landlord mitigated.
If the landlord takes the property off the rental market, thus failing to mitigate, you owe him nothing from that day forward.
What you have to understand, though, is that breaching your lease is a bad thing which could result in financial consequences to you. In your case the landlord is giving you a choice of essentially paying a lease breaking fee or taking your chances that he decides to re-rent, instead of sell, and then sues you for the rent from the time you leave until the time he re-rents, which could take anywhere from weeks to months depending on the rental market and you would certainly owe for that period.
And, as T-M notes, he can sell it as a rental property with you as occupant and have the new owner assume your lease. Doing that would obligate you to the remaining year of your lease.
It's up to you whether you want to accept the amendment or just move out and take your chances.
One thing I suggest is not to leave the amendment open ended. Offer, instead, to limit your obligation to, say, one month's rent.

