Gemara

(Mishnah - R. Akiva): We do not invest money of Aniyim (the poor) in order to reap profits, lest an Oni come, and there will not be what to give him.

Bava Metzi'a 70a (Rav Anan citing Shmuel): We may lend orphans' money on Ribis.

(Rabah bar Shila): One may invest orphans' money on condition that they are apt to profit but not to lose. (This is Avak Ribis, which is normally forbidden mid'Rabanan. Regarding orphans, Chachamim permit it.)

Question: What do we do with orphans' money?

Answer #1 (Rav Yosef): We leave it in Beis Din and give them each Zuz when they need it.

Objection (and Answer #2 - Rabah): The principal will dissipate! Rather, we find a man who owns pieces of gold; we buy pieces of gold with their money, and invest it with him, on condition that they are apt to profit but not to lose.

We cannot use a man who owns only identifiable things. Perhaps they are a deposit. The owner will give a Siman and retrieve his deposit!

Objection (and Answer #3 - Rav Ashi): If we do not find such a man, will we allow the orphans to consume the principal?! Rather, we find a man whose property is uncontested; he must be trustworthy, knowledgeable of Torah law and does not accept to be excommunicated. We invest the money with him in Beis Din, on condition that they are apt to profit but not to lose.

Erchin 6a: (Beraisa): If one said "this coin is for Tzedakah," it is permitted to change it before the Gabai (overseer of Tzedakah) received it, but not afterwards.

Question: R. Yanai was a Gabai, and he used to borrow Tzedakah and repay!

Answer: That benefited the poor. He did so to induce people to give more Tzedakah (he could tell them that there is no Tzedakah money to give to the poor)!

Rishonim

The Rif and Rosh (Bava Metzi'a 31a and 7:50,51) bring the Gemara in Bava Metzi'a.

Rosh (Sof 51): We invest the money with him in Beis Din, in order that the fear of Beis Din will be on him.

Beis Yosef (YD 160 DH u'Ma'os): The Mordechai says that it must be in Beis Din to allow Ribis mid'Rabanan. Talmidei ha'Rashba suggest that perhaps Beis Din must authorize lending the orphans' money.

Nimukei Yosef (Bava Basra 67a DH Omar ha'Mechaber): The Ra'avad says that an Apotropos must get Reshus from Beis Din or the orphans' father in order to get a share of the profits, even the part due to his money.

Rambam (Hilchos Matanos Aniyim 8:4): Whether one said 'this coin is Tzedakah' or 'it is upon me to give a Sela to Tzedakah', he can change it for another coin. Once it comes to the Gabai's hand, he may not change it. If the Gabayim want to change the (small) coins for Dinarim, they may not. If there are no Aniyim to whom to give them, they may change the coins for other peoples' coins, but not for their own.

Rambam (5): If the poor will profit by the money staying in the Gabai's hand, for it will enable him to make others give, he may borrow it and repay it. Tzedakah is not like Hekdesh, which is Asur b'Hana'ah.

Rambam (Hilchos Nachalos 11:1): We do not require an overseer for money that orphans inherited from their father. Rather, we find a man with Idis (highest quality land). He must be trustworthy, knowledgeable of Torah law, and was never excommunicated. We invest the money with him in Beis Din, on condition that they are apt to profit but not to lose. The orphans benefit from the profits. If he does not have land and gave to them a security of fragments of gold without a Siman, this suffices. The judges decide how to divide the profits: a third or a half, or even a quarter for the orphans, if this is good for them. If they cannot find such a man, we feed them little by little from the money, until we buy land with it and appoint an overseer.

Poskim

Shulchan Aruch (YD 259:1): Before Tzedakah money comes to the Gabai's hand, one may lend it to himself or to others. Once Tzedakah money comes to the Gabai's hand, one may not lend it, to the giver, to others or the Gabai. If the poor will profit by the money staying in the Gabai's hand, for it will enable him to make others give, he may borrow it and repay it.

Question: In the Reisha, we do not allow lending out Tzedakah, even for the sake of profit!

Answer (Taz 2): We permit here when he is sure that he can collect from the Tzibur immediately when the poor will need, without any hindrance.

Gilyon Maharsha (4): If Levi appointed an overseer on all his property and on the money he was Makdish (for Tzedakah), the overseer can change it, for this is like money that passed to orphans; it did not yet come to the Gabai. Similarly, if one was appointed to invest Tzedakah money for profit, he is like people of the city, and has more autonomy over the money.

Rema: This is because Tzedakah is not like Hekdesh. One may benefit from it. We do not invest Tzedakah money destined to be distributed, lest Aniyim will come, and there will not be money to give to them. We may trade money for money. We may invest Tzedakah that is not destined to be distributed, rather, the principal will remain and Aniyim will eat the Peros.

Source (Sefer ha'Terumos (46), cited in Beis Yosef DH Kasuv b'Sefer): We do not seek profit with money of the poor, just (we may exchange it for other) money and similar things, so that it will be ready whenever Aniyim come. Hagahos Maimoniyos says that this refers to Tzedakah money destined to be distributed, for the poor rely on it. We may invest Tzedakah that is not destined to be distributed, rather, the principal will remain and Aniyim will eat the Peros. People do so.

Shulchan Aruch (CM 290:8): We do not require an overseer for money that orphans inherited from their father.

Rema: Some require an overseer also for money, for he will supervise it better than Beis Din.

Gra (16): This opinion explains that the Heter to invest orphans' money is through an Apotropos.

Shulchan Aruch (ibid.): Rather, we find a man with Idis. He must be trustworthy, knowledgeable of Torah law who was never excommunicated. We invest the money with him in Beis Din, on condition that the orphans are apt to profit but not to lose. If he does not have land and gave to them a security of fragments of gold without a Siman, this suffices. The judges decide how to divide the profits: a third or a half, or even a quarter for the orphans, if this is good for them. If they cannot find such a man, we feed them little by little from the money, until we buy land with it and appoint an overseer.

SMA (19): The Shulchan Aruch means, a third or a half for the borrower, or even (three fourths for him and) a quarter for the orphans.

Rema: Some say that just like the Apotropos can invest the money with others, he can accept it himself (on these terms), as long as it is in Beis Din, to avoid suspicion.

SMA (20): One who lent and received a security may rent it to others, but not to himself, due to suspicion. Here, Beis Din is the father of orphans. Since Beis Din is aware, it is as if the orphans themselves gave over the money.