Assalamoalykum.
 
1: Is this allow to eat/drink the food water leftover (kafir ka jhota) by a Kafir or if in a plate Muslim and Kafir can eat together?
 
2: If a person unintentionally say Qasam that I will not do some thing (without being intention) is Kaffara is due if he do that thing?
I hope you will answer me. JazakAllah Khair
 
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Walaykumsalaam

1.       If a non Muslim does not have the impurity (Najaasat) of any Alcohol or Haraam products (etc) in his mouth, then his saliva (Jhoota) is considered clean (Paak). However this will have affect on the Taqwa of a Person.

سُؤْرُ الْآدَمِيِّ طَاهِرٌ وَيَدْخُلُ فِي هَذَا الْجُنُبُ وَالْحَائِضُ وَالنُّفَسَاءُ وَالْكَافِرُ إلَّا سُؤْرَ شَارِبِ الْخَمْرِ وَمَنْ دُمِيَ فُوهُ إذَا شَرِبَ عَلَى فَوْرِ ذَلِكَ فَإِنَّهُ نَجَسٌ وَإِنْ ابْتَلَعَ رِيقَهُ مِرَارًا طَهُرَ فَمُهُ عَلَى الصَّحِيحِ كَذَا فِي السِّرَاجِ الْوَهَّاجِ. (فتاوى هندية ص23 ج1)

(Fataawa Mahmoodiyyah [Vol. 5 Pg.237])

2.       No there will not be Kaffara. This will be considered يمين لغو (Yameen-e-Lagw).

Allah has mentioned Yameen-e-Lagw in the Quraan:

لَا يُؤَاخِذُكُمُ اللَّهُ بِاللَّغْوِ فِي أَيْمَانِكُمْ (سورة المائدة: 89)

Allah does not hold you accountable for your Lagw (ineffectual) oats.

 However this is a bad habit that should be avoided.