Gordon participates in his employer's HSA$/$HDHP$.$ On a monthly basis, Gordon contributes $$100$ to his account; his employer made a lump$-$sum contribution of $$500$ to each participating employee's account at the beginning of the year. If Gordon leaves the employer mid$-$year, what happens to the $$500$ employer contribution?

a$.$ Gordon must pay back the full $$500$ to his employer.

b$.$ Gordon must pay back a pro$-$rated share of the $$500$ to his employer.

c$.$ Gordon keeps the full $$500.$

d$.$ The employer may, at its option, deduct up to $$500$ from Gordon's final paycheck.