In common speech, your definition of "disposable income" is perfectly acceptable.

https://www.merriam-webster.com/dictionary/disposable%20inco...

https://dictionary.cambridge.org/dictionary/english/disposab...

Note that the Oxford Dictionary uses GP's definition, probably to distinguish themselves from Cambridge :)

https://en.oxforddictionaries.com/definition/disposable_inco...

It's probably a case of a word used incorrectly, but so often that ends up assuming a different meaning.