I pray you are well.
1) If one muslim says to a second Muslim to clear the mortgage so theres barkat/blessing on the house etc. And the second Muslim says 'how dare she say that to me'.
2) Does the second Muslim saying the above become a kaafir? The second Muslim has no intention of kufr and believes in the shariah law of interest.
May Allah bless you.
الجواب وبالله التوفيق
As per the situation described in your question, by stating the words mentioned, the person does not become a kafir(nonbeliever).
واللہ اعلم بالصواب