Buying a house on contract, seller has a VA loan.. Subject To?

[Emily Cook]

Sellers agreed to sell me the house on contract for 6 months. Their mortgage is roughly $1000, so I offered them $1200/month and put $2000 earnest money down, held by title and returned to me at closing. My contractor said no problem on a 90 day (max) turn around. 

My only concern at this point is if their loan calls the balance due mid construction. I am approved for a hard money loan, and a conventional 20% down purchase. But the purchase price agreed upon does not factor in hard money costs, and if it is mid construction that could pose a problem for the conventional loan approval. 

Anyone have insights on this?? THANK YOU!