Is it permissible for someone entrusted with delivering money to a creditor to deduct from it an amount equal to his debt to that creditor, without informing the original owner of the money?

The trustee must deliver the trust in full to its owner, and it is not permissible for him to dispose of it except with the owner's permission; for betrayal of trust is one of the major sins.

Therefore, you are obligated to deliver the full amount as it is, and it is not permissible for you to take anything from it. The issue of "seizing a right" does not apply here, because you are an agent of the paying person in delivering his debt, and not an agent of your indebted friend.