Collums doesn't allow the creation of money without a paper trail. To increase an account, the client "buys" an account item as usual. To decrease an account, the client buys something using their account money.
Increase an account total
Create a new payment type called "Account adjustment" or similar.
Go through the normal process to take a deposit or payment on account of the value that you'd like to increase the account by.
Take payment as usual but use the Account adjustment payment method (you may also wish to leave a note on the invoice explaining what you've done and why).
Ensure that the email & SMS payment notification checkboxes are unticked so that your client doesn't receive a notification of the transaction.
Reduce an account
Create a new product called "Account adjustment" or similar with 0% tax.
Go to the POS and create an invoice containing this item.
Change the price of the item to match the amount you'd like to reduce the account by.
Take payment using the "Account" payment button. (you may also wish to leave a note on the invoice explaining what you've done and why).
Ensure that the email & SMS payment notification checkboxes are unticked so that your client doesn't receive a notification of the transaction.