If the amount of credit that needs to be deducted from your account is 50 credits and the member only has 30 credits in their account, then I would expect all 30 credits to be deducted from their account. It should not go into a negative balance. If there is no credit in their account, I would expect the transaction to be skipped. If there is 0 credit in their account, I would expect this transaction to be skipped.