חשב לעשות מצוה ונאנס נחשב כאילו עשאה

The Har Tzvi brings an interesting rayah. The Gemara in Bava Kama (16b), writes that Yirmiyohu Hanavi was very upset with Klal Yisrael who had been serving avodah zarah. He went to the Ribbono Shel Olam and asked that He shouldn’t give them any reward for any mitzvos that they do. He added, that even when they do the mitzvah of tzedokah, Hashem should make them stumble and give money to people who don’t really need tzedokah (frauds). The question is, even if they gave tzedokah to a fraud, they had good intention and they intended to do a mitzvah? We see from here, that if in the end of the day one doesn’t fulfill a mitzvah, he doesn’t receive any reward.

However, the above doesn’t seem to fit with the well-known rule of:חשב לעשות מצוה ונאנס נחשב כאילו עש אה - “if one has intention to fulfill a mitzvah, and in the end something happened and he didn’t manage, Hashem looks at it as if one did the mitzvah”?

The Har Tzvi quotes a Shevus Yaakov (2:10), who answers, that when a person does everything which is within his power to do, and he does it right, i.e., he buys tefillin from a sofer mumcha [expert scribe], had them checked manually and by a computer, and then somehow a few years later it is discovered that they are pasul, then we say עשה חשב לעשות מצוה ונאנס נחשב כאילו, as one did all he could.

However, if one doesn’t do everything he can, then one doesn’t get rewarded, unless, he actually fulfills the mitzvah. Therefore, in the case of tzedokah, if one ends up giving to a fraud he doesn’t get rewarded as if he would have looked into it more, he would have discovered that it was a case of fraud. Similarly, if one finds his tefillin pasul after many years, it would depend on the type of mistake one finds, as to whether we can apply the rule ofחשב לעשות מצוה ונאנס נחשב כאילו עשאה or not.