PF: Is it possible to terminate a Postdoctoral Fellowship before the planned end date?

Early termination of the Grant Agreement is possible. You can check here (p. 18, #37).

The host institution needs to put in place an amendment to terminate the grant. The amounts corresponding to unspent person months will have to be returned to the European Commission. The funding corresponding to person months already implemented does not have to be returned.

Link to IT How To (step 2).