Anyway, I suspect that this tenant has allowed a newly-acquired boyfriend to move in, against the lease, which stipulates that just the tenant is to reside in the apartment.The other more senior residents in the building have seen this new boyfriend practically on a daily basis, coming and going, for about two weeks now.

And if you did agree to that stipulation, what legal recourse is your lender able to take against you to recoup this $25k penalty?

The language of the contract will stipulate how default is handled.My advice would be to get an extension signed and allow extra time for a buffer. 

Property management and real estate laws differ depending on what state -- and even city -- you live in. ...Write an explicit and easy-to-understand contract. ...Include all the stipulations that you require. ...Include details regarding the deposit. ...Consult with an attorney.

I would offer him a year lease with other stipulation in it.

Maybe I'll look into a cash out refi; I wonder if there are any stipulations regarding the number of properties.

You could have had a stipulation where if he got paid x% by a third party,  and you would pay the difference between the 5%.

Any stipulations?

Your agreement will need to stipulate the ownership share.  

The PSA stipulated that one commercial lease had to be in effect at the time of closing.