Absolutely....the owner of the debt may hire anyone to collect it or sell it to anyone for any amount at any time. It is his asset. It does not change your obligations in any way.
The fact there is this whole huge collection business everyplace which do exactly that, (buy debt at a discount and make money by collecting it all)...should be a hint to you...and finally, why would you think what the person you owe does with his stuff and in no way changes what your agreement was, should change what you promised to do?