Planet Mercury has a global magnetic field of internal origin. Its low intensity, 340 nT at the equatorial surface, indicates that the Hermean dynamo is special as much stronger fields are to be expected. Here, we suggest a feedback dynamo model where Mercury is embedded in an external field which is generated by the Chapman-Ferraro current of the small magnetosphere of Mercury. This self-generated ambient field has a pronounced influence on the dynamo action. Based on a kinematic α-Ω-dynamo model we show that the feedback dynamo has two solutions, a Hermean-type solution with a weak magnetic field and an Earth-like one with a strong field. The Hermean solution is that one where the external field opposes dynamo action, while an Earth-like solution results as the Chapman-Ferraro field is negligible in the dynamo region.