Black & white film - a classical pianist with a beautiful daughter is given an injection by a suitor that mutates his hands, ending his piano career... Suitor/enemy offers the antidote only if the pianist consents to give his daughter in marriage. Serum eventually turns pianist into evil beast, who eventually kills suitor/ enemy (?) Antidote is found, playing career resumes

Sounds like this:
The Monster Maker (1944 full length sci-fi horror movie! Classic!) - YouTube (http://www.youtube.com/watch?v=BCKigNXiATs)
... A mad scientist infects a concert pianist with acromegaly in hopes of convincing the daughter to marry him in exchange for the cure...