That isn't quite how Bitcoin works. Transactions use specific unspent prior transaction outputs (from the "UTxO" set) as inputs; there is no concept of "remaining funds". However, you're mostly right: if the attacker controls the private keys related to the original transaction then they can insert a conflicting transaction and make the original one permanently invalid on that chain. However, an attacker who does not have the private keys to sign a conflicting transaction cannot prevent the transaction from eventually being confirmed unless they control over 50% of the hashpower.

If you're concerned that the person you're receiving funds from might be willing to attempt a double-spend then you should wait for a suitable number of confirmations before considering the funds successfully transferred. However, if they control over 50% of the hash rate then there is no suitable number of confirmations that would make the transfer safe.