If you have anything in writing from the employer asking for the $2500 you paid earlier, that would make your case strong. Also if you have anything now where he is asking for the remaining $2500 then you can use it to report to DOL. If not, then get in writing and them tell him that these emails are going to DOL. i am sure he understands the consequence of that.
Nothing against you, but coz of such employers and candidates others are suffering at each step of the immigration process now. JUST STOP THIS.
PS: I never knew the H1b fees increased to 5k. huh.