[CutX]

Not sure but if you mean the money under your health bar,
that just won't change if you change the values of variables.
You have to use GivePlayerMoney(playerid, - or +);

i'd recommend you do
ResetPlayerMoney(playerid);
and then
GivePlayerMoney(PlayerInfo[i][pCash]);//after you've taken the taxes


but if you did mean the money on the bank,
you should do:
banksys[dCash] -= tax;
instead of
banksys[dCash] += tax;
and you shouldn't use taxes here:
PlayerInfo[i][pCash]
since youre taking it only from their bank account.
Otherwise they'll get it withdrawn twice, hand + bank