Origin of “quid” in its sense of a sovereign or guinea
Solution 1:
The OED is less confident than some other sources:
Origin uncertain; perhaps [from] classical Latin quid ‘what', reinterpreted within English to refer to (monetary) means or wherewithal. (My emphasis.)
Solution 2:
According to Milroy J. and Milroy. L, "Authority in Language" (1985),
[...] quid pro quo probably gave rise to the slang "quid", a unit of money which varies with the context in which it is used.
And the plural is often the same as the singular. (same reference)