Hi,
The setup time for the flop doesn't change. The data still needs to be stable 100ps before the clock edge hits the flop.
Assuming the flop that created the data has zero delay on its clock line, you have a "fast" clock flop talking to a "slow" clock flop. The data can now arrive 40 ps later and still meet setup requirement.
The net effect is that the flop "appears" to have a setup time of 100-40=60ps.
In this scenario, fast-to-slow, I would watch out for hold violations.