on the nail (of payment) without delay.
☞ The origins of this expression are uncertain. It may be related to the obsolete phrase to the nail, meaning 'to perfection' or 'to the utmost', which derived from the habit of sculptors giving a finishing touch to their work with a fingernail, or to joiners testing the accuracy of a joint in the same way. A North American equivalent is on the barrelhead.
2011Alex HarrowellYorkshire Ranter They would surely have been likely to insist on payment in cash on the nail, rather than promises of future side-deals that would likely never be fulfilled.