Oh man, I am so sad the collection agency didn't get their fee! (...blowing my nose...)
Unless the collection agency purchased your debt from the creditor, they don't deserve anything.
Where you messed up is not getting an agreement from the creditor to remove any credit report from your records, in exchange for you paying off the bill. Big mistake. Now they can leave it there.
Remember, the creditor/collection agency doesn't give a rip about you or your life. They only want their money. That is the ONLY leverage you have over them. So in the future, insist IN WRITING that if you pay the bill they clear your credit history. Otherwise, why in the world are you paying them anything? You will still have bad credit, plus you are broke!
Get armed! This is war!!!
All you can do now is demand that a notation be placed on your credit report that the bill is now PAID IN FULL. It may still show late payments (not good) but it will help a little.
Good luck, read the links I've supplied for sample letters and other info.