It takes a little time to contact them, find out if they want to stay, provide written notice of the rent increase, produce the document and then actually stay on them to get it signed.   

If it is something that is going to resolve itself quickly and it was a good offer it may be worth waiting.If the contract specifies a loan approval contingency date and they are not able to produce a formal loan approval from a lender by that date the contract should also be null and void.

The job did not produce any document/receipts for obvious reasons, and common sense would tell me this is not possible.

I was trying to produce something similar using Zillow's APIs but I couldn't find parameter that would give the HOA fee.

This would actually be a great opportunity to sell the duplex...and buy a large apartment building since you have so much equity and they tend to produce better income . 

Let's chat in private as I do not want to seem to solicit to anyone and everyone on a public forum but in general our innovate model that installs in applications where plumbing is not separated probably runs $200-300 per apartment unit ($400-600 for an entire duplex).

(c) A water supply approved under applicable law, which is:(1) Under the control of the tenant or landlord and is capable of producing hot and cold running water;(2) Furnished to appropriate fixtures; and(3) Connected to a sewage disposal system approved under applicable law and maintained in good working order to the extent that the system can be controlled by the landlord.

Clean water and local produce. 

Our first went better than expected, currently producing $1100/month net cash flow, around $50k equity upon stabilizing.

So now i would have an income producing property owned outright worth 30k.