一手硬, 一手軟 the carrot and stick

軟硬兼施，恩威並重的意思。

e.g. The boss is using the carrot and stick approach to the negotiation with his subordinates, promising to increase welfare and threatening them with dismissal if they do not stop strike. 

(參考成語)
play good cop and bad cop

e.g. In negotiation with the worker, the two managers are playing good cop and bad cop.