My question involves labor and employment law for the state of: District of Columbia

I have just resigned from my place of employment for the last 7 years effective December 31st. Per our policy, I will be paid accrued vacation time on the 31st. However, we are now in a disagreement on the payout amount because early in the year I decreased my LOE to 50%. My understanding is that my salary remained the same, but that I would be compensated at 50% LOE. During those months, I also only accrued 50% of my vacation benefit which is accrued in hours (I have 266.88 hours). So my understanding is that the outstanding amount due to me is based on a hourly wage calculated by my full salary. HR is now arguing that I will only be paid based on earnings, which of course are half of what I would be paid if I were full time. We have had a long argument about earnings vs salary and I want to make sure I understand what the law says before I get into a fight with them on this (its a substantial amount of money -- almost 10K between what I believe I am owed and what they say they owe me). My sense is that they are arguing that instead of being paid 50% of my previous salary, that my current "salary" is actually 50% what it was, which is completely inaccurate and there is no documentation whatsoever that states that I took a salary reduction at any time.

I'm not going crazy here, right? What are my rights?