Every listing has a minimum offer price – the lowest price a buyer can offer to the seller for the app in question. The site minimum is $500, but sellers are free to select their own minimum offer price above this. With every offer made, buyers are encouraged to explain the full context of why they are offering the given amount. A buyer may cancel their offer at any time BEFORE the offer is accepted by the seller. Once an offer is accepted, both parties are bound to commence the transaction.
As a courtesy, Apptopia will inform you if any offer you make is below another existing offer on the listing. Regardless, you will still be free to make your offer.