Is a person rewarded for a deed that was done purely for the sake of Allah, then it was mixed with some ostentation or regret after its performance?

Scholars have differed on whether subsequent showing off (riya') after the completion of a sincere good deed invalidates it or not. Some say it invalidates it, while others say it does not. Ibn al-Qayyim mentioned that there is a disagreement on this matter. Ibn Muflih stated that the presence of showing off after finishing an act of worship does not invalidate it. Al-Ghazali opined that it is unlikely for something that occurs after a deed to invalidate its reward; rather, what is more fitting is that one is rewarded for their deed and punished for their showing off. Accordingly, showing off after a deed does not invalidate it, but its doer is punished for it. One must beware of showing off and the stratagems of Satan, which may drive a person to abandon good deeds.