Citadel of Cassiopus green/purple doors and keys

[Toybox]

Nope, there is only one key for two doors (originally to get rid of leftover keys from the previous Sections). The idea is that you must choose what will be more useful at that point, for example a backpack vs. an extra life. (If this makes it easier for you, think of that money as an 'extra life' instead of 'extra money').