I haven't taken the next steps with the banks yet so I'm not sure how happy they would be with the down payment coming from a debt LLC. 

@Christopher Phillips   maybe i'm reading this wrong, or it works differently in NY, but a 6% seller concession normally means the seller is giving the buyer 6% of the purchase price to use however they want in the transaction - be that for closing costs or principal reduction or down payment, whatevs.A seller typically doesn't have a say of what can and can't be included in the borrowed amount, that's up to the lender and the loan program. 

We are wondering if the property price for my SFR will go down once the construction is complete as I have never been in this situation before.

It will help with your exit strategy...down the road.

Not as much as multifamily but still more than STNL single tenant or 2 to 4 tenant type retail center properties.The larger centers they live or die by the anchor tenant so really have to know if that tenant owns or lease the space, when it was last re-imaged inside and out, and what sales they do per sq ft year over year and how they have been trending.Larger retail centers have defaults from smaller tenants with covid right now to work through so offering should have reserves built in for some not making it so buy in cap rate on existing NOI might go down in the short term as tenants are lost and tenant replaced.

The buyers came down to $50,000.

Keep your head up...

Going off the top of my head so might be some holes...1. have them supply an application (most generic rental apps ask the right questions)2.

Lower down payment and keep the current as a rental or sell the current property and larger down payment on the new one?

In high season, I can rent each almost as much as a 4 bedroom house does but off season I do much better as people coming down would prefer a small condo on the beach.