Responsa for Bava Metzia 181:1
לא שאדם רשאי להמיר אלא שאם המיר מומר וסופג את הארבעים א"ל הא מני ר' יהודה היא דאמר לאו שאין בו מעשה לוקין עליו
Not that one is permitted to make an exchange, but that if he did the exchange is valid, and he receives forty [lashes]!<span class="x" onmousemove="('comment',' Tem. 2a. This refers to Lev. XXVII, 33; neither shall he change it (sc. the consecrated animal): and if he change it at all, then both it and the change thereof shall be holy. The first clause of the passage states that all have power to exchange, and then it goes on to say that that does not mean that one may exchange, but merely that his action is valid, the substitute too becoming holy, and that his action is punished by flagellation. Now, this offence consists only of speech, and hence this Mishnah refutes Resh Lakish's view that speech is an unsubstantial action. ');"><sup>1</sup></span>
Teshuvot Maharam
A. Lending charity-funds on condition that the lender share in the profits but not in the losses, is forbidden. The Rabbis who said (B. M. 70a) that a person lending money on such a condition is called "wicked" made no distinction between rich and poor. Since a poor person is not permitted to lend his money on condition that he share in the profits but not in the losses, we are not permitted to do so for him.
Teshuvot Maharam
A. Lending charity-funds on condition that the lender share in the profits but not in the losses, is forbidden. The Rabbis who said (B. M. 70a) that a person lending money on such a condition is called "wicked" made no distinction between rich and poor. Since a poor person is not permitted to lend his money on condition that he share in the profits but not in the losses, we are not permitted to do so for him.
Note: The following introduction precedes the answer: Please excuse me for the brief response. For the last two weeks I have been confined to bed and have lost the taste for food. I wrote to you a lengthy response regarding the lending of charity-funds, and sent it to you through a messenger named Marvel. I forgot the lengthy argument and discussion. I remember, however, to have agreed with your reasons and arguments, and to have added the following ….
SOURCES: L. 425; P. 476. Cf. Agudah B.M. 73.