a normal part of payment processing - to block the access to the sum for the particular transaction if such is available before continuing with the rest of the procedure - transfer etc.

This can happen with disputes involving debit cards whereby the bank withholds the amount for payment until the dispute is settled. (The bank will either pay the amount, or if there is a dispute, will withhold the payment until it is settled.)

http://www.finweb.com/banking-credit/know-how-to-handle-cred...
The Fair Credit Billing Act gives you the right to withhold payment to the credit card company for defective merchandise or inferior service, as long as certain conditions are met. If you do withhold payment, you may only withhold the amount of the item in dispute. Other non-disputed purchases that are included in the statement must be paid for as usual. If payment is withheld for a disputed item, the credit card company cannot enter any derogatory information into your credit file, nor can they report the account as delinquent while the charge is being disputed.

Say your credit card limit is $1,000 and you spend $600 to buy something. The card company, upon authorizing your payment, holds the $600 credit as unavailable, so that you can only spend $400 on your card until you pay the balance.