A factor price taker pays labor a wage such that w = MPL x P. And if a firm is perfectly competitive, P = MR = MC. Thus, it follows that w = MPL x MR = MPL x MC.
Suppose that Sean works for Sunny Sky Co, a perfectly competitive firm producing solar lights. Sean was paid $5,000 but found a better job and quit Sunny Sky. Since nothing else changed, Sunny Sky’s total revenue decreased.