First you are correct this is a business not a charity or non profit or community service, save community and charity work for the part of you life that you have dedicated to those causes, which by the way is a must if you want to be a complete self actualize person. 

He would be better off forming a charity so he could at least take a tax credit or something for not taking rents.

@Maria Amaro this sounds like a personal matter for the buyers and you aren‘t runnIng a charIty.

The GC isn't running a charity and neither should you be.So if you told them or had a contract with them to do it at cost (without any stipulation as to what cost truly is) you would probably be able to argue that you should get the actual true expenses plus some admin fees and your time on the job without profit. 

You're not a charity, you're running a business.

Put the property in a trust for the benefit of a charity.

Maybe pick a charity in your area and give the money to them in your referral's name.  

Quick suggestion here would be to make them tangentially related to income- for example you want to give 5,000$ to a charity of your choice.

I do not want to fall behind market rents, I want to contribute to setting market rents and I do not want to turn my business into a charity by supplementing tenants rent.

If I buy the house I can't just break the lease, it must be honored, but I don't want a year of someone paying 30% market rent.It must be nice to be able to run a charity Tom, should I offer to pay their utilities and perhaps buy them a car, because they may be great people?