Money and time are not interchangeable at all.
One can take 1-2 hours of his time, when he has time free.
One cannot give 1-2 hours of money, when he has to pay for his rent, bills and family.
There is noone who can decide to work one hour more at his workplace and get paid for it, then donate the money to charity.
There are plenty of people that can make this exact choice. I'll work extra hours on occasion when my job is willing to allow us overtime, but not requiring it. Admittedly it's generally my stove broke and I need X dollars to replace it, not me donating the money.
So yes, I could absolutely work one more hour and get paid for it at the end of the pay period.
Hopefully you're also saving for retirement, rainy day funds, etc. - some of these bills are flexible.
> There is noone who can decide to work one hour more at his workplace and get paid for it, then donate the money to charity.
Anyone freelancing can take on more client work. Save less and retire later. Kick more ass at work to get more promotions or other job opportunities. I could take shorter breaks between jobs.
It's not perfectly interchangeable, but the whole point of work is to trade time for money - "not interchangeable at all" is a strong enough phrase I'd argue work-for-pay alone contradicts it.