Balances cannot be adjusted generally, they are only the sum of the transactions that affect a user.
So to adjust a balance, send fiat to that user.
There is however the possibility to use the Airdrop extension to add balance to a group of users. This can be based on user roles (so you could give some fiat to the admin for example).
If you want the user to pay the admin, the user must initiate an internal transfer with [wallets_move].
If you want to write your own code, you can also initiate transactions via the JSON API (e.g. from the browser) or via the PHP API (from your theme or another plugin).
Let me know if this answers your question. If not, what is your use case?
with regards