5 CFR §839.812 — Will my employer get a refund if I had a qualifying retirement coverage error and the employer retirement contributions for my new retirement plan are less than what was already paid?

No, if you were erroneously in CSRS, CSRS Offset, or Social Security-Only, then a correction of a retirement coverage error will not reduce the employer retirement contribution owed. Also, the FERCCA states that an employer may not remove from the Fund FERS employer contributions when correcting a qualifying retirement coverage error under this part.