Is the value of what the owner of the goods takes based on agreement, or does it have a fixed value?

If the owner of merchandise says to another: "Sell this merchandise for such-and-such [a price], and you will have such-and-such [an amount]," or "Sell it for such-and-such [a price], and whatever exceeds that is yours," then there is no harm in that, and the seller shall have what was agreed upon. It has been narrated from Ibn Abbas and Az-Zuhri that this type of sale is permissible.