The (Pre-)Master-Secret is unique per TLS session. If you want a third party to be able to decrypt a specific pcap with this TLS session but not anything else it is safe to give the party this secret, as long as the session is not reused anywhere else.
Note that a TLS session can span multiple TCP connections. To make sure that the session will not be reused anymore by the client it is usually enough to close the client, i.e. the client usually does not preserve session information over restarts. But you should also make sure that the third party has no access to other data captures from the same TLS session apart from the ones they are explicitly allowed to analyze.
Since a TLS session is always between a specific client and target you don't have to worry about the TLS session reused between the same target and another client or between the same client and another target. But "target" does not necessarily mean a single server - it is not uncommon that a domain has multiple servers and that these share TLS session information.