All you need to free yourself from slavery is a couple of pints of beer
I read Ivanhoe for the first time translated into Spanish, and am having a heck of time finding the material in any English editions for reference.
Scott made his Saxons drink ‘ale’, not ‘beer’. In doing so he was following the Saxons’ own usage: the word ‘beer’ was “rare, except in poetry, and it seems to have become common only in the 16th century” (OED). So you are looking for this piece of dialogue:
“Thou art an ass,” replied one of the thieves; “three quarts of double ale had rendered thee as free as thy master, ay, and freer too, if he be a Saxon like thyself.”
Walter Scott (1819). Ivanhoe, chapter XI.