Reference for Nedarim 96:7

תנן אמרו חכמים כל מתנה שאינה שאם הקדישה תהא מקודשת אינה מתנה כל לאיתויי מאי לאו לאתויי הא מילתא דשדיא בכיפי לא לאתויי לישנא בתראה דשמעתיה דרבא:

R. Ashi demurred: But in the case of a sudarium, who tells you that if he retains it, it is not his?<span class="x" onmousemove="('comment',' I.e., though in fact it was only a symbol, and usually returned, yet it may be retained; but here it was not intended that the son should have possession at all but merely to be the medium of transference, for if his grandson would not be a scholar, the estate was to revert to his second son. ');">⁷</span>

Jastrow