Specifically, when setting a negative interest rate on e-money, the central bank would let the conversion rate of cash in terms of e-money depreciate at the same rate as the negative interest rate on e-money. The value of cash would thereby fall in terms of e-money.
2 See De Bortoli, D., Galí, J. and Gambetti, L. (2018), “On the empirical (ir)relevance of the zero lower bound  constraint”, UPF Working Paper No 1594. 3 4/5  BIS central bankers' speeches  3 See Heider, F., Saidi, F. and Schepens, G. (2018), “Life below zero: Bank lending under negative policy rates”,  Review of Financial Studies, forthcoming.