Rambam - 1 Chapter a Day

See Chapter 9, Halachah 1.

If, however, a time when the produce must be returned is established, the loan is forbidden, as stated in the following halachah.

With this phrase, the Rambam excludes a market price in villages that fluctuates, as stated in Chapter 9, Halachah 4.

And thus there is a certain resemblance to a loan at interest, for the lender received something of greater value for his loan.

Note the Siftei Cohen 162:10, which states that this law applies even when the borrower does not have the cash on hand to purchase the produce.

The rationale is that although a formal transfer of ownership was not made, it can be said that the lender acquired the produce and that it increased in value while it was in the lender's possession. Although this is a legal fiction - and even on that level, not entirely correct, as reflected in the subsequent clause - our Sages nevertheless permitted such a transaction. Since there is no Scriptural prohibition involved, they did not enforce the safeguard they established when there was plausible vindication for the loan.

It is not permitted, however, to specify a date when the produce is due to be repaid, as stated in the conclusion of the halachah.

The rationale is that he is allowed to borrow the first se’ah because of the se’ah he possesses. After he borrowed one se’ah, he has two se’ah, and so he can borrow two se’ah against them. This pattern can be continued to include any amount of produce. Again since there is no Scriptural prohibition, the Sages were lenient (Maggid Mishneh).

I.e., the produce that the borrower receives as a loan may be far more than the amount he possesses.

See the Siftei Cohen 162:9, which writes that if the two principals know that a market price was established, it is acceptable even when they do not know the price itself.

Lest the price rise and the transaction resemble interest. The rationale - is that the pattern of the increase and decrease of commodities is generally foreseeable. One can assume that until the harvest, the price will be high and after the harvest, the price will be low. Hence, by specifying a date, the lender will pick one that will enable him to lock in a profit. Even though the borrower may repay the debt before it is due, it is unlikely that he will do so (Maggid Mishneh).
The Ra’avad differs with this ruling and maintains that as long as the person possesses the produce or a market price is established, the loan is permitted, even if a specific date is set. The Maggid Mishneh refutes that argument, stating that if this was the case, the Mishnah (Bava Metzia 5:9; see the following halachah) would not have to state explicitly that it is permitted to give a loan of produce until one’s son returns. That would be obvious.
The Ramah (Yoreh De’ah 162:3, as interpreted by the Siftei Cohen 162:11) rules that a due date may be set as long as the borrower has the option of returning the wheat before the due date if he desires.

I.e., if the market price for wheat was a zuz for each se’ah, taking a loan of wheat means taking a loan for a zuz.
The Shulchan Aruch (Yoreh De’ah 162:1) states that these laws apply not only to produce, but to merchandise and/or any article other than the prevailing local currency. The Shulchan Aruch (loc. cit.:2) also suggests an option for the lender: He or another person should give the borrower a small amount of produce. Once the borrower possesses this small amount, a loan of a much larger amount can be made.

If, however, the borrower returned the produce after it increased in value, he may not expropriate the difference from the lender, because the prohibition against such loans is only “the shade of interest,” which is not expropriated through legal process (Chapter 6, Halachah 1; see Siftei Cohen 162:1).

In which instances, it is permitted to make a loan of produce.

As explained above, we assume that the lender has protected his own interests, and the price will rise by the time the debt is due. Hence, such a loan would resemble interest.

I.e., he may not make a loan of produce with a set time for repayment that is in the distant future. This applies even if he possesses wheat at this time.

I.e., a loan of produce may be made for a short time, provided the borrower possesses that produce. Significantly, the Mishnah (Bava Metzia 5:9) mentions the opinion of Hillel, who forbade one woman from borrowing a loaf of bread from a neighbor unless a financial equivalent was established. Although his opinion is not accepted as halachah, it does give us a barometer on the safeguards our Sages felt were necessary regarding at such loans.

I.e., in violation of the prohibition mentioned in Halachah 2.

Thus, in neither of these situations will the lender have profited.

The term “entered the field” as used in this halachah refers to beginning to work the field and thus establishing his rental of the field as an unalterable agreement.

I.e., that the owner can still give a loan after the sharecropper took possession of the field.

Even though the sharecropper has entered the field, since he has not supplied the seed, he is not fulfilling his portion of the agreement. Therefore, the owner has the right to re-negotiate the agreement.
In such a situation, providing the seed is not considered to be a loan, but rather is leaked upon as part of the sharecropping agreement. For example, if the sharecropper borrowed five se'ah of wheat, instead of being obligated to pay the owner 25 se'ah, as the other sharecroppers are, it is as if this sharecropper agreed to pay 30 se'ah. This is permitted because it is considered part of the rental agreement and not a loan (Kessef Mishneh).

And the owner wanted to depart from the local custom and require the sharecropper to provide the seed.

I.e., the owner is still able to negotiate with the sharecropper and make any demands he desires. If the sharecropper isn't happy with them, he can seek another field.

I.e., we are speaking about a situation where the rental agreement was already finalized and the owner was obligated to supply the seed. Nevertheless, he sought – and for some reason, the sharecropper agreed - to have the seed he provided considered to be a loan, and not his responsibility (Kessef Mishneh).

Since the sharecropper agreed to repay the debt at the time of the harvest, a date for repayment has been established. Hence, the loan is forbidden, as stated in Halachah 2.

As is any other person, as stated in Halachah 1.

In Halachah 2, leniency was granted even when the borrower possessed only a small amount of produce. Here, by contrast, he must have an amount equivalent to the debt. The rationale is that here a sale is involved, and the reckoning must be exact (Siftei Cohen 163:2).

Even though the price of wheat rises and the lender profits on the transaction.
The Ramah (Yoreh De’ah 163:1) states that we accept the borrower’s word if he says he possesses the wheat, and we do not require him to supply proof.

The Beit Yosef (Yoreh De’ah 163) rules that this prohibition applies even when the borrower possesses the money with which he could purchase the produce.

The rationale is that since the transfer into the commodity is considered to be “the shade of interest,” the borrower is not required to pay it, as stated in Chapter 4, Halachah 6 (Maggid Mishneh).