[[Wikipedia:Clan_Buchanan]] states without clear attribution that Walter (14th/7th Chief)'s son by Isobel Graham, Patrick, m. "the daughter of the Earl of Argyle;" whereas, the Guthrie Smith, John (1896) text makes no statement about Patrick's wife. Furthermore, all of the evidence on Lady Margaret (Campbell) points to her only having one marriage, and that was to George Seton. Nevertheless, none of Margaret's sisters appear to have married a Buchanan.
Wikipedia cites Strathendrick and its Inhabitants from Early Times (Smith, 1896) in general discussion of Walter and his son Patrick's relationship to Clan Buchanan; however, by my read, Smith (1896) is mute on identifying Patrick's spouse. It's appears to be a good source on the Buchanan line - the other source that the Clan Buchanan page lists as pertaining to Walter and Patrick includes no sources and basically is an historic narrative with an impossible to decipher pedigree.
Thus it appears that Patrick's wife has unknown origins, and that she is definitely not Lady Margaret, the daughter of the Earl of Argyle (UNLESS: am I misreading this statement?): what Smith (1896) does say means something that I am missing:
On 22 Jun 1486, gift by Colin, Earl of Argyll, to Walter Buchquhanan of that Ilk, of the marriage of John Campbell of Ardfinglace.
Smith (1896) immdediately cites Montrose Writs in ''The Lennox'', Vol 11, p. 127. The statement would perhaps seem to suggest that a daughter of Walter married John Campbell of Ardfinglace(?)
Note also that even Burke appears to be silent on who Patrick's wife was.
 https://digital.nls.uk/95335539 inclusive of p. 286 - excerpted under CC 4.0