Is it permissible to hire someone to fast two consecutive months for an expiatory fast?

The general rule for expiations (kaffarat) is that they are to be performed at one's leisure, unless their cause was unlawful, in which case they become immediately obligatory. Thus, if the expiation is for intentional or semi-intentional killing, or for dhihar (a form of divorce), or for breaking the fast of Ramadan, it is immediately obligatory. However, if it is for accidental killing, it is obligatory at one's leisure, though it is always preferable to expedite it. As for hiring someone to fast an expiation for another person, this is impermissible because fasting is a physical act of worship and deputization is not valid therein.