I have a test setup to sell 1 HQM for 1000 blood.  If I make the purchase it shows I can buy it because I do have a balance.  However it does not take the blood from the balance.  I can keep buying and not spend anything.

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I also have setup if I sell 1 HQM for 900 blood.  This allows me to do the sale but does not change my balance.

I am trying to look into issues with the plugin today.  If the machine is not set to bottomless, it will transfer the money to the owner.  If you are the owner then it would make sense that a transfer yields 0 change.  For me, as soon as I applied the flag and made a purchase, the money was deducted.