Trademark principles are a bit more complicated than you might imagine.
What sort of name are we talking about? (see this discussion:
https://www.namepros.com/threads/domain-portfolio-vs-udrp-trademark-question.1145945/#post-7320006 )
Are we talking about something like "Microsoft-Word" and "Microsoft-W" here? Or are we talking about something like "Autos-BMW" and "Autos-B"? Is "Name" or "xyz" the more dominantly distinctive part of the mark or is distinctiveness of this mark relatively narrow that absent the entire combination, a string under consideration would not be likely to be deemed similar enough?
What matters, as an initial point, is whether the domain name is "confusingly similar" to the mark. In other words, is it likely that the "Name-xyz" and/or its components sufficiently distinctive that the domain "Name-X" is likely to be considered related to the "Name-xyz" mark or not.
These kinds of determinations come down to very specific facts, and don't operate one some set of algebraic rules. That's why most of these types of questions on Namepros have no answer that is reliably applicable to your facts, but do provide an opportunity to discuss the various sorts of questions that one would want to ask in order to get anywhere near an answer that might apply to your particular facts.
Now, a big clue in the probable answer to that inquiry is the fact that you seem to think it is similar enough or likely to be considered confusing enough, that it would be of value to approach them to try to sell a domain name. Because, pretty obviously, if "Name-x" was unlikely to be considered to have something to do with the mark owner in "Name-xyz" why would you think it would be worthwhile to sell them the domain name in the first place?
A better question to untangle this thought process is to ask the converse question - Are there lots of other people/companies who are likely to be interested in buying this domain name? If so, then why not write to all of them, and let them know that they are not being singled out.
The reason that matters is explained in this UDRP decision over Cambridge.com, which I defended. "Cambridge" can refer to many things, and the domain registrant's broker had contacted dozens of entities who may have been interested in that geographically descriptive term, both in the greater Boston area and in the UK...
The Chancellor, Masters and Scholars of the University of Cambridge v. Kirkland Holdings LLC
Case No. D2015-1278
https://www.wipo.int/amc/en/domains/search/text.jsp?case=D2015-1278
"As to the offer for sale made to the Complainant by a broker entrusted by the Respondent, the Respondent claims that the Complainant was only one of the possible interested parties contacted by the broker and that, therefore, the Complainant was not specifically targeted by the Respondent.
...
Fourthly, the Respondent could also demonstrate that there is an economic rationale underlying the offer for a possible transfer of the disputed domain name to a third party. Indeed, whereas the term “Cambridge” refers to different and relevant geographical locations, it is plausible that there may be other market players potentially interested in acquiring the disputed domain name. The consequence of this competition for the disputed domain name is the elevation of the price of its transfer.
Finally, a general offer of sale in 2014 or 2015 does not by itself support the Complainant’s conjecture that the Respondent registered the disputed domain name four or five years earlier “primarily for the purpose of selling, renting, or otherwise transferring the domain name registration to the complainant.” Policy, paragraph 4(b)(i)."