If you obtain a PFD agreement in writing, pay it. Per the agreement, the collection will be removed and you should see some increase on your score. You have the agreement in writing as backup.
If you can't obtain a PFD in writing, don't pay it. Attempt to PFD again or wait it out. Upon paying a collection without having it removed, the collection becomes "recent" and will drop your score significantly.
OP, is this collection the only negative item you have on your CR, or do you have others such as late payments still on there?
A lone major negative item falling off if there is nothing else negative present can typically raise FICO scores 70-80 points.