Responsa for Nedarim 88:1

ודלמא אתי לעשורי מן החיוב על הפטור ומן הפטור על החיוב

but perhaps he will come to tithe from [produce] that is liable for [produce] that is exempt, or vice versa?<span class="x" onmousemove="('comment',' The tithe could be separated from one lot of produce upon another (of the same species), providing that both bore the same liability. E.g if one harvests his two fields, he can take From one the tenth of the combined produce. If, however, he separates a tithe of one field, thus freeing the rest, he cannot take another tithe from the same for the second field. Similarly, if he has two lots of corn, one liable to tithe by Biblical law, and the other only by Rabbinical law, so that by Biblical law it is really exempt, he may not separate from the one for the other. Now it has been explained here that according to R. Jose, so long as no stranger has taken possession, it is not hefker by Biblical law even after three days. and consequently Biblically liable. But by Rabbinical law it is hefker, even if the original owner re-acquires it. Nevertheless, as explained on p. 139, n. 5, the Rabbis ordered that he shall tithe it. Thus, in this respect, the Rabbis restored it to Biblical law. But the owner, being told that it is hefker, may regard the liability to tithe as merely a Rabbinical measure, and therefore, if he has any other corn which is only Rabbinically liable, separate from the one, which is really Biblically exempt, For the Biblically liable, or vice versa. ');">¹</span> — He is told, 'When you tithe, tithe for it out of itself.'<span class="x" onmousemove="('comment',' Only in this respect is it regarded as hefker even if the first owner resumes possession. ');">²</span>

Teshuvot Maharam