Responsa על סנהדרין 143:6

Q. I heard that you permit the lending of charity-funds on condition that the lender share in the profits, but not in the losses, resulting from its investment by the borrower. Such a transaction is forbidden by Rabbinical decree; why, then, should it be permitted in connection with charity-funds? Signed: Hayyim b. Machir.
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.