It says you're not to look at it.

Look at the white rat and count your money! On your honour don't look!' The others responded nobly.

They looked at the white rat till they quite stared him out of countenance, so that he went and sat up on his hind legs in a far corner and hid his eyes with his front paws, and pretended he was washing his face.
Cyril stooped again, busying himself with the other bootlace and listened for the Psammead's further instructions.
'Go in,' said the Psammead, 'and ask the price of lots of other things.
Then say, "What do you want for that monkey that's lost its tail--the mangy old thing in the third hutch from the end." Oh--don't mind MY feelings--call me a mangy monkey--I've tried hard enough to look like one! I don't think he'll put a high price on me--I've bitten him eleven times since I came here the day before yesterday.

If he names a bigger price than you can afford, say you wish you had the money.' 'But you can't give us wishes.

I've promised never to have another wish from you,' said the bewildered Cyril.
'Don't be a silly little idiot,' said the Sand-fairy in trembling but affectionate tones, 'but find out how much money you've got between you, and do exactly what I tell you.' Cyril, pointing a stiff and unmeaning finger at the white rat, so as to pretend that its charms alone employed his tongue, explained matters to the others, while the Psammead hunched itself, and bunched itself, and did its very best to make itself look uninteresting.