Far from being the work of any one individual, the phrase Soapy Tenner has existed since the agricultural revolution, when Jethrow Tull - fresh from inventing either the seed drill or the spinning jenny - staggered into a local house of ill repute, and demanded a 'soapy titwank' to wash the filth that clung to his nether regions.
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.
I bet you a soapy tenner Jethrow Tull didn't invent the spinning jenny, fool.