Baba Basra 54b - Dina D'malchusa between Jews

It is clear from the gemara that the concept of dina d'malchusa applies between a Jew and a gentile, so that even when the issue will ultimately boil down to a conflict between two Jews, we would still follow the dina d'malchusa since the issue began with a Jew and a gentile. In our gemara Reuven purchased property from a gentile and after paying but before receiving a contract Shimon made a kinyan in the property. Following dinei yisroel the property belongs to Shimon, but following dina d'malchusa it would belong to Reuven. The goy is essentially out of the picture because he got his money and has no more claim on the land, and the argument is only between Reuven and Shimon. Nonetheless, the gemara holds that we would follow dina d'malchusa to say that the goy did not relinquish rights to the property upon the receiving of money so that Shimon's chazaka was invalid, therefore the property belongs to Reuven.

The Chazon Ish (C.M. likutim 15) points out that from our gemara we see that if the issue begins between a jew and a gentile, even though it is 'megalgel' into a conflict between 2 jews, we follow dina d'malchusa. He also writes that the reverse is true as well. If the issue begins between 2 jews, then they are bound by dinei yisroel even if they are living in a secular country that has other laws. But this is only true if the government doesn't care that the Jews are following a different system. So long as the government is not makpid that the Jews follow their system such as in monetary issues where the side that would win in secular law but not in Jewish law can always be mochel their claim, all conflicts between Jews must be judged according to Torah law. The Chazon Ish is medayek this from the R"I cited by the Shita mikubetzes (baba basra 55b) that even if the secular courts require 40 years for chazaka, they are not makpid about Jews following a 3 year chazaka system since the מערער can be מוחל on his claims, therefore they are obligated to follow dinei yisroel. However, if they would violate this din by going to secular courts and it would be judged by the secular courts it would be binding, since they used the power of the government to take it away from the machzik and give it to the me'ar'er.

Based on this, if for whatever reason the government was makpid that the Jews follow the secular system rather than the Jewish system in a particular monetary law, the dina d'malchusa would seem to trump the din torah (since the malchus has the ability to take away money from the person who was zocheh in din torah and give it to the other). The Chazon Ish (#3) writes this explicity: In a case where the courts charge a tax for contracts and are therefore makpid that contracts be done in secular courts, even contracts between 2 Jews are bound to dina d'malchusa since we consider it like an issue between a jew and a goy (the goy being the government who wants to charge the tax).