If you request a refund for a specific pack you have purchased, the app store will return you the money and the currencies acquired will be removed from your account. This is a measure of insurance to make sure that everyone has the same opportunities in the game.
However, if you have already used the purchased items and if you don't have enough coins/diamonds to keep a positive balance, that will not prevent the purchase to be reversed, and therefore, you will end up with a negative currency balance.
Then, you will receive a message stating that you must acquire a new pack in order to play, since it is not possible to keep playing with a debt in your account. Keep in mind that we are not responsible for providing refunds for purchases, so if you have negative currencies and you haven't requested a refund, please contact Apple or Google support (depending on the platform you use).