[meekaaku]

The above looks like $70 was taken from cash and deposited to bank. Thats not whats happening in the real world. Since cash is fungible, one could ignore that. But if it was cheque + bank-transfer or something else that leaves a record, then that wont work. Or when you include VAT/GST/Sales tax. Or when you pay a loan monthly payment of $1000 that needs to be split to principal and interest. etc.

[yobbo]

Yes, it is best if transactions correspond to verifiable events. Complicated situations can be modelled by letting events be represented by their own "accounts", in a separate charter.

In your example, you might let the monthly payment be represented by its own account, with three transactions (bank payment, interest, principal).

The purpose is to make the database constraints enforce double entry verification. If you don't need this, it can be made simpler as you suggest.