As a practical question, this hinges on many issues that are beyond our strategic-political expertise. Although one’s initial response might be that one obviously may not do anything that would help murderers such as ISIS, that may be simplistic. Consider that one who follows this approach to its logical conclusion would have to get rid of his car because ISIS is financed significantly by oil sales and usage affects the market. Since we have no idea to what extent ISIS sales of Jewish artifacts are an issue, this response is a theoretical analysis – not in any way practical guidance for any individual or group.
First we must consider whether there a mitzva to save these artifacts. There are two possible mitzva reasons to “redeem” them. One is to save holy articles from being disgraced. Another is to save Torah information for the Jewish people. People are often motivated to buy such items because of their desire to possess coveted Judaica, but it is difficult to consider a personal reason like that a mitzva.
Saving holy scrolls from being disgraced is recognized as a value for which it is worthwhile to pay a halachically significant price. Accordingly, it is permitted to violate certain Rabbinic laws of Shabbat in order to save from a fire holy writings that have sufficient sanctity to require geniza,[1] whether they are halachic sifrei Torah, remainders thereof, or even other Torah writings.[2] On the other hand, we do not find sweeping leniencies or an obligation to seek out such items to save. It therefore does not seem that this factor can overcome such an obvious wrongdoing as helping a terrorist organization.
The second consideration, the matter of saving vital information, comes up in the discussion of pidyon shvuyim.[3] The mishna[4] says that despite the great mitzva of redeeming captives, the Rabbis prohibited paying more than the captive’s “market value.” The apparently accepted explanation is that paying excessively encourages the taking of more captives. Accordingly, Tosafot[5] asks how it was permitted for R. Yehoshua ben Chananya to pay an exorbitant price to free a certain youngster to whom he took a particular liking.[6] One of Tosafot’s answers, which the Shulchan Aruch[7] accepts, is that it is permitted to pay a high price for someone with the potential to make great Torah contributions. The same logic should apply to redeeming a valuable Torah work.
However, “redeeming” Torah works is apparently not included in formal pidyon shvuyim, the goal of which is to alleviate human suffering.[8] In some ways, this may lessen the mitzva to redeem them. On the other hand, if sefarim do not fall within the formal realm of pidyon shvuyim, they also are not within the formal Rabbinic prohibition of overpaying. Thus, if one wanted to extend the prohibition involving ransom to any especially dangerous “seizer of Torah scrolls,” we would say that neither the mitzva to pay something nor the prohibition to pay too much applies formally. The absence of a formal prohibition, however, does not imply that one should not use moral common sense.
The price that people are willing to pay for valuable Judaica often has little to do with its practical importance for Torah information, but rather relates to its historical, sentimental, or even artistic value. In the case of a terrorist organization, it seems inexcusable to pay even the “going rate” for such an artifact if it means helping an “organization” like ISIS, which perpetrates atrocities.
Let us put things in perspective. The Rabbis sometimes forbade commerce that might be used to further sinful activity.[9] On the other hand, the Rabbis were careful not to forbid more than society is able to handle; there is a limit to how many things we can boycott (remember our example above about cars). Yet, in a case as stark as the one you raised, the spirit of the law suffices to preclude buying even important holy objects when there is a rational concern that people could be put in mortal danger as a result. Only in exceptional cases might one contemplate that the cost-benefit consideration makes redemption morally feasible.