This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 牮 is uncertain. It visually combines 代 and 牛, but the original semantic or phonetic motivation is unclear.
No clear meaning is known, so a mnemonic is not possible. The character looks like a substitute (代) cow (牛), but this is not a confirmed meaning.
For セン, imagine a substitute (代) cow (牛) paying a sen (銭) coin as a toll: sen -> セン.