Талмуд к Маасер шени 2:6

MISHNAH: Of profane and Second Tithe coins that were strewn around, what one collected goes to Second Tithe until complete, the remainder is profane⁸⁴One assumes that the profane and the tithe coins are indistinguishable. What is collected first has to be sanctified as Second Tithe money by a declaration, and what had been Second Tithe coins and now ends up in the profane part is then automatically redeemed by the profane coins which ended up in the tithe part.. If he mixed them together and took out fists full, it is in proportion⁸⁵If the coins are not picked up as they fell but first are swept together with a broom to form a heap in which all coins are mixed, the coins taken out are separated into heave and profane according to the proportion heave and profane had before the accident. According to Maimonides (in his Commentary and his Code, Ma‘aśer Šeni 6:1) the proportional split in this case holds even if some coins were lost. In this case also, a declaration of substitution has to be made.. That is the principle: What is picked out is for Second Tithe and what is mixed together by proportion⁸⁶For all Second Tithe, not only coins. This is spelled out more clearly in the Tosephta, 2:4..
If a tetradrachma of Second Tithe was mixed up with a profane one⁹¹And it is not known which of the two coins is dedicated as Second Tithe. The farmer does not want to take both coins to Jerusalem., one brings coins in the value of a tetradrachma and says: The tetradrachma of Second Tithe, wherever it may be, is exchanged for these coins. Then he chooses the better of the two and exchanges it for them since they said, one exchanges bronze for silver in an emergency⁹²But not silver for silver. When the silver coin, the standard legal tender in the entire Roman empire, is spent in Jerusalem it is usually first converted into local bronze coin. Therefore, this exchange is legal under certain circumstances. But silver coins under normal circumstances are never exchanged into other silver coins and, therefore, such an exchange is never permitted. The bronze coins should not be kept as such because non-local bronze coins may not be accepted at full value in Jerusalem., not that it should stay so but that he should exchange them for silver.
The House of Shammai say, a person should not turn his tetradrachmas into gold denars but the House of Hillel permit it. Rebbi Aqiba said, for Rabban Gamliel and Rebbi Joshua I turned their silver coins into gold denars.