SQL database design: many to many relationship between ‘accounts’ and ‘transactions’ tables such that a transaction can have ‘from’ and ‘to’

I would simplify your design by creating an additional AccountsTransactions table with composite key from Accounts and Transactions tables and add a new column send_receive_indicator or transaction_type (S – Send/R – Receive) on Transactions table (assuming that each transaction is either send or receive type of transaction). This way you can control your many-to-many relationship on that AccountsTransactions by introducing that new transaction_type column in your query.