I dont think it can be solved as 5 is a odd number and all the given jars are even number. Any + or – between two even numbers produces only even so it cant be solved.

Source:- Four different jars of 2, 4,6 liters

Boy will simply propose her by saying I LOVE YOU .
its not mentioned that the girl has to understand the boys proposal , the girl has problems (blind ,deaf ,dumb blah blah ) not the boy .

References:- Boy will simply propose her by saying I LOVE YOU .