Let's say there is one house remaining in the bank. Alice wants to build it on the light blues (normally $50/house) and Bob wants to build it on the dark blues (normally $200/house). Per the rules, it goes up for auction:
When the Bank has no houses to sell, players wishing to build must wait for some player to return or sell their houses to the Bank before building. If there are a limited number of houses and hotels available and two or more players wish to buy more than the Bank has, the houses or hotels must be sold at auction to the highest bidder.
Is there a minimum price this house needs to auction for? Can Bob get this house for $100 despite the normal purchase price for a house on the dark blue houses being $200?