I always say, no good deed goes unpunished; meaning “letting” tenants just move a few things in, now makes you liable for their belongings if something were to happen i.e. flood, robbery, fire etc… AND those things can and do happen, then their renters insurance probably doesn’t start until the 1st of September and you said their lease doesn’t start until the 1st of September.Do the right thing, refund back the previous tenant IF the new tenants move in early, and also change the lease date and the new tenants need to pay from that date.

I have absolutely no experience. 

So, for properties that offer stays no less then 28 days, you would get a landlord or LTR insurance policy.

Maybe you can approach with a combination of selling the lowest performing or least desirable asset(s) and utilizing some for cash out towards reinvestment? 

The money raiser has no idea what is going on. 

They had no lease in place, but have been paying on time. 

Also, in a mainstream asset class like value-added multifamily, I see no reason to take a risk on a sponsor that doesn't have full real estate cycle experience or that lost anything more than a small amount of money (and prefer no money lost).

It's best to get a local attorney's advice on this; no strangers.

My realtor and her manager said “No, I am not aware of this being an acceptable means to unilaterally change the terms of the lease.

No offense, but if you can't evaluate deals in your own city, why are owners hiring you to manage their assets?