A solicitor's bill of costs may include costs payable in discharge of a liability properly incurred by him on behalf of the party to be charged with the bill (including counsel's fees) notwithstanding that those costs have not been paid before the delivery of the bill to that party; but those costs—
shall be described in the bill as not then paid; and
if the bill is assessed , shall not be allowed by the costs officer unless they are paid before the assessment is completed.