You would need to use policy nat/static nat in order to differentiate when traffic will use that IP address only when going to the tunnel, something like
access-list VPN permit ip
static (inside,outside) Y.Y.Y.Y acces-list VPN
and the crypto map will use that Y.Y.Y.Y as the source of the vpn traffic.
Now one little catch here, if you are going to use a single ip address, then PAT is required and the config will change, causing this not being bidirectional (only replies to traffic from your inside network will come back, not traffic originated from the remote network) for PAT use
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...