Following training we provided in Anchorage recently, a senior scheduler pulled me aside with a perplexing P6 problem. He had inherited the schedule for a hospital in northern Alaska that had been originally assembled by a cast of now-long-goners. By and large the schedule was reasonably solid, but some quirky behavior somewhere in its structure revealed itself every couple months, throwing pay apps out of balance. Try as he might, (and he's a solid scheduler), the errors kept recurring. One month all the pay items would balance, and the next there would be some math that just didn't add up.  It boiled down to our needing to adjust the amounts billed in some now-closed financial periods. The solution turned out to be simpler than we expected. Here is the solution to how to update closed financial periods in p6:

1) In the Details view, we opened the Resources Tab

2) We customized the Resources Tab to include columns for Units and Costs from the (now-closed) Financial Periods that were incorrect.

3) Consulting the manual, we entered the DIFFERENCE. in the Actual Units and Actual Costs for the period. (P6 will not overwrite the original value, but it will post an ADJUSTMENT to the existing total for the closed financial period)

4) Bob then followed the usual routine to Close the Financial Period, but rather than close the Current Period suggested by P6, he selected the prior closed period instead that we’d updated. It worked.

Takeaway: carefully examine and adjust ALL components of the cost equation, and not just the obvious ones.  It took us a couple tries to get it correct.