The page
http://kittycats.biz/transactions.php shows your K$ transactions and balances.
At the top are two lines:
The K$ "balance" shows K$ from sending cats to the Menagerie. This is the ONLY balance which can be used to obtain Menagerie Collectibles.
The K$ "bonus" shows K$ from payments of L$ to your dock. This CAN NOT be used to purchase collectibles.
The total of these two is the total K$ you have on hand which can be used to purchase other items, such as food or milk. Normally, these purchases draw from your "bonus" until it hits zero, then draw from your "balance".
Yes, it's a mess, illogical, and bears only a passing resemblance to normal accounting. But it's what we have and I doubt it'll ever be fixed.