The motive for paying in gold was--avoidance of discovery.

A cheque can be readily traced.

So can banknotes.

But gold is not easily traced.
Therefore the man who paid Collishaw fifty pounds took care to provide himself with gold.

Now then--how many men are there in a small place like this who are likely to carry fifty pounds in gold in their pockets, or to have it at hand ?" "Not many," agreed Mitchington.
"Just so--and therefore I've been doing a bit of secret inquiry amongst the bankers, as to who supplied himself with gold about that date," continued Jettison.