He did drive for Sauber-Mercedes in sportscars before he came to F1. His team mate was Heinz-Harald Frentzen who was, by all accounts, generally faster than Schumacher.

Anyway, I digress, I doubt this as Schumacher wouldn't race unless he thought he could win and Merc-GP didn't want to pay Button the going rate, a seven times world champion will want less ? But his mate Ross Brawn is there so nothing would surprise me these days