Casting a spell to reunite with my lover, will that interfere with his own free will?
In all types of magic, there is a fine line that one shouldn't cross! With that said casting a spell or conducting a ritual to bring back your lover is fine, it is the manner in which the work is done that is important.
When I work with a client to reunite him or her with a lover, I work to resurrect the energy of the love that is already there!
Each case is different; for example, casting a love spell to bring a specific person into your life is fine, there are ways to deal with karma, and to side step a negative backlash, again, it is all about the intent, and will of the practitioner, and of course skill plays a large part in a successful outcome to any spell or ritual.