Power of attorney is not permitted, but I did have one lender who blessed off on a corporate resolution for a stateside entity member to have signing authority for the business.

Once you start negotiating with a tenant, all of your authority and credibility goes out the window. 

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You have refused to sign this form along with your ACH authorization form & W9 form. 

Definitely get with their broker and if you don't get satisfaction there, contact the state regulation authority for real estate licensing to file a complaint.

In my sub-markets, it is hard to get the Section 8 authority to move up to market rates.  

If the tenants try an end around by calling him directly, he tells them he cant do it without us authorizing whatever work it is.

I send the tenants a bank authorization form and set up a recurring receipt in QBO that charges them on the first of the month.

Now that co says their ins wont cover a claim for repairs bcuz they didnt authorize it.

The fact you learned the children are the ones selling the property through a neighbor and this was not clear in the sales agreement suggests the proper authority might not have even existed to execute the agreement on behalf of the estate in the first place.