Someone gets an aliyah to the Torah and promises "chai" to tzdaka. He just doesn't remember if he said "one time chai", "two times chai" "three times" etc. The greatest possibility is 18 times "chai". How much does he have to give??
Apparently, he must give 18 times chai. The Rambam [Matnos Aniim 8-3] and Shulchan Aruch [Yo"d 258-3] say that when one is unsure how much he vowed, he must give the largest amount that he is in doubt about.
We can explain this din based on the מחלוקת of the רמב"ן and רשב"א [cited in the ר"ן נדרים ז,א ד"ה ולענין הלכה] who hold ספק ממון עניים לחומרא - if one is unsure whether one money is coming to the poor, he must be stringent and give it to them, while the Ran holds that if one is not sure if money belongs to the poor he may keep it and not give it to them - ספק ממון עניים לקולא.
So the Rambam seems to be learning like the רשב"א that ספק ממון עניים לחומרא so he has to give the maximum while according to the Ran one would in our case give the least possible amount that he is obligated.
If the gabbai remembers then apparently we can trust him because עד אחד נאמן באיסורין. Now even though we pasken that מספק he must be מחמיר, we can't FORCE him to be מחמיר [Chasam Sofer and Noda Bi-yehuda]. We must then ask if due to the testimony of the gabbai, we have the right to force him.
[מאוצרות הגר"א גנחובסקי זצ"ל]
