各位版上的高手大家好，
想请教下面句子中 but 的词性
As part of the transaction, but not included in the written agreement,
the plaintiff gave the defendant $400 which was used to purchase and
install a new pump.
我的看法是，as是准关系代名词，修饰后面整句"the plaintiff...pump."
然而but词性是什么呢?
是连接词、副词还是介系词?
请版上个位高手帮忙了，谢谢。

副词，跟however相当。btw，这边的as个人认为是介系词，表“作为”。

谢谢大大，言简意赅。