A Soapy Tenner is a £10 note that you will never actually see but still gives a modicum of weight to your argument by it's very presence.
From the example given we can see that even though the chance of the tomorrow being sunny is quite high, there is a possibility that it won't. You should therefore NOT risk your real tenner, but use your Soapy Tenner which has the built in option of non-payment.
The origin of the Soapy Tenner is a regular forum user at www.visordown.com you would regularly bet £10 on the outcome of various discussion and never pay.
The phrase originates from the online UK motorbike forum 'Visordown', specifically a user called 'Soapy' known for making bets with no intention paying anything.
A Soapy Tenner says you are wrong.
The quick witted whore replied that she'd want a 'soapy tenner' before she let him ream her jubblies with his filthy love-wand, and Tull agreed.
after shooting a sticky load into her hair, he proceeded to knee her in the crotch and run off without paying. The phrase 'Soapy Tenner' came to mean a tenner you're never likely to see.