What is the meaning and ruling of sale by exchange?

Question:
A man asks about the ruling on sale by exchange?

Answer:
This statement is vague.
Q: The description is that the merchant brings the goods and says: I do not want money from you; if you sell, give me the money?
A: No, it is necessary to sell it either for a known amount or immediately; so that it is done with clarity. Either he says it is immediate and he may demand it whenever he wishes, or else for a known term.
Q: He says: Whatever you do not sell, return it to me, is that permissible?
A: It is necessary to have a known term.
Q: So it is said: Is it left as a precaution or is it not permissible?
A: It is not permissible; it is necessary to finalize the sale.
Q: And what is this called?
A: They call it: selling on the condition of his loss, that he may return it whenever he wishes, and this is among the invalid conditions.