Reference for Bava Metzia 156:11

השוכר את החמור והבריקה מאי והבריקה הכא תרגימו נהוריתא רבא אמר אבזקת

But perhaps there too, the reason is that he [the donor] gave it only [that it be used] for purim and not for any other purpose?<span class="x" onmousemove="('comment',' Consequently, when the poor man wishes to divert it to some other use, it is not a case of robbery, but simply that it is not his for that purpose, and is deemed never to have come into his possession. ');">¹¹</span>

Jastrow