The Sages forbade rabbinical courts to sit in judgment or mete out punishment on Shabbat. Similarly, they prohibited betrothals, marriages, divorce, yibum (levirate marriage), or ĥalitza (levirate divorce) out of concern that people would end up writing (Beitza 37a). It is similarly prohibited to redeem a firstborn son on Shabbat, because the redemption (pidyon ha-ben) involves a transfer of money, which resembles commerce. If the 31st day after birth coincides with Shabbat, the redemption is done the following day, Sunday. It is also forbidden to vow, consecrate, or dedicate items or their value to the Temple on Shabbat, as this transfers ownership of the objects to God, as it were, and resembles commerce. However, one may pledge to give charity, because such a commitment involves no act of acquisition. If one did buy, sell, or perform any of these actions on Shabbat, the transaction is effective (m. Beitza 36b; SA 339:4).
Teruma, ma’aser and ĥalla may not be set aside on Shabbat, because setting them aside resembles dedicating them to God. Additionally, it looks like one is fixing or improving the produce (Beitza 36b; MT 23:14). If one made a mistake and unknowingly set them aside, the remaining produce may be eaten on Shabbat. In contrast, if he did this knowingly, although the action is effective and the remaining produce may be eaten, no Jew may eat it until after Shabbat (m. Terumot 2:3; MB 339:25).
One who is concerned that he will not have time to separate teruma and ma’aser before Shabbat may recite the formula for the separation before Shabbat (but without the berakha). By doing so, he has begun the process of separation. Then, after Shabbat begins, he may tithe in the normal fashion and recite the berakha. This procedure may also be followed if one is concerned that he will not have time to set aside ĥalla.
Only the owner of the produce may separate teruma and ma’aser on Shabbat by means of the procedure described in the previous paragraph; it is not effective for anyone else. If a guest is concerned that his host may forget to tithe, he may ask the host before Shabbat to appoint him a shali’aĥ (proxy) to tithe on his behalf. The guest may then recite the formulation before Shabbat and perform the actual tithing on Shabbat (m. Demai 7:1, 5; MT, Laws of Tithes 9:7-9).