Is it permissible to take a known quantity of a commodity and sell part of it, then return the remainder and pay only for the sold part?

If he buys a specific quantity of honey for a deferred price, and stipulates that he can return what he is unable to sell, then the sale is valid and the condition is void. However, if he buys an unspecified quantity based on what he can sell, then the sale is void due to the unknown quantity and price.