The PB limit (120 in 20 team leagues, 115 in 24 team leagues) is enforced exactly once each year. It is enforced on the day when you cut your roster to 27 after the rookie draft and before the free agent draft. At any other time your PB value can be as high as you want it.

And for the doubters out there this does actually come into play on a very few teams. There is one team in one of my leagues that currently has a PB value over 150 for next season and he has taken no steps (so far) to alleviate the problem. Another team in one of my leagues is over the limit now but I think he will easily get under it when he makes his cuts.