Sheikh Muhammad al-Manî`î, professor at Umm al-Qurâ University
When someone, in response to some good turn on your part, says to you: “May Allah reward you with good.” (jazâk-Allahu khayran), the appropriate thing to say is something like: “And you as well” (wa iyyâka) or “And may He reward you.” (wa jazâka)
At the very least, he should say âmîn in response to the supplication that is being made to Allah on his behalf.
A reply of “that’s alright” or “that’s okay” is not appropriate. However, there is no sin or serious objection for doings so.
As for the misgivings that you are having that your reply implicates a rejection of Allah’s blessings, these are the mere whisperings of Satan.
A person who suffers from such baseless misgivings should blow to the left three times and seek refuge with Allah from Satan three times.
He should also take care to recite the supplications of the morning and the evening and to keep your tongue moist with the remembrance of Allah. Otherwise, he subjects himself to the danger of being susceptible to Satan’s whisperings, similar to the misgivings that you describe in your question.
And Allah knows best.
Source: Islam Today