In kṣayya and jayya there is substitution of ay for e only then when the sense is that of 'to be possible to do'.,
The roots क्षि and जि before the affix यत् 3.1.97 assume these forms when meaning to be able to do the action denoted by the verb. As शक्यः क्षेतुं = क्षय्यः (क्षि + य = क्षे + य); so also जय्यः ॥ Why do we say when meaning \to be possible to do\? Observe क्षेयं पापं, जैयो वृषलः ॥ Here the meaning is that of 'necessity'.,