Responsa for Bava Metzia 114:9
כלל ופרט וכלל אי אתה דן אלא כעין הפרט מה הפרט מפורש דבר המטלטל וגופו ממון אף כל דבר המטלטל וגופו ממון
at four se'ahs per <i>sela'</i>, whilst it subsequently stood at three se'ahs per <i>sela'</i>]. As we learnt: If one contracts to supply flour at four [se'ahs per <i>sela'</i>], and it [subsequently] stood at three, he must supply it at four; at three, and it [subsequently] stood at four, he must supply it at four, because <i>hekdesh</i> [always] has the upper hand.<span class="x" onmousemove="('comment',' Shek. IV, 9. The contractor received payment in advance, and fixed the price before the market price was out. Now, if the purchaser were a laymen, this would be forbidden as usury, (infra 62b); as, however, the bargain is with hekdesh, it is permitted. According to this, the passage does not refer to a loan at all. ');"><sup>9</sup></span>
Teshuvot Maharam
A. This is undoubtedly prohibited. But since the lending of charity-funds at a definite rate of interest became such a widespread custom throughout the Kingdom that the prohibition of the practice would not be heeded even if the law became known we refrain from publicising this prohibition. This opinion is expressed, however, in order to urge the compliance with this prohibition in the future (but R. Meir would express no opinion as to the necessity of repaying the interest already collected).
SOURCES: Pr. 73; L. 478. Cf. L 234; Cr. 101, 109; Tesh. Maim. Mishpatim, 14; R. Asher, Responsa 13, 8; Agudah B.M. 73.
Teshuvot Maharam
A. B should not give the ten quarter-marks to the poor of that community, since it would constitute unlawful usury. If, however, B earned profit with the ten marks, he must give part of the profit to the poor of another city, since one is not permitted to profit from charity money.
SOURCES: Pr. 999.