It seems that you are concerned about funds from a transaction that was sent to you after a backup of your private keys has been made. That's not something to be concerned about.
The private key is only needed to spend any bitcoins sent to your Bitcoin addresses. When you have imported the private key, all unspent transactions to that private key's corresponding Bitcoin address can then be spent, regardless of when those transactions occurred (i.e., including transactions that were sent after your backup was made).
Now keep in mind that if you are the sender of a payment, change goes to a new bitcoin address in your wallet. A backup will contain a number of addresses (default is 100) in a key pool which are used for transactions that follow when you made your backup, however if you are frequently sending funds you should frequently be making additional backups.