Assuming the paper money fails.

You have a gold ingot. 100 Gram. with a current value of £1000. (But the paper actually is worth nothing)

So you go into a shop and want £100 Pounds worth of groceries.

How do you pay ?

You would be relying on the shopkeeper giving you the balance back in gold.

i.e he has to give you 90 gramms of gold.

How will this work in practice.

And how will the echange rate be worked out. There has to be one.