Generally, a debt collector of consumer debt may not demand any more that what is legally due. Most consumer credit contracts have “acceleration clauses” where if you are deemed to be in default, the entire amount is accelerated and now due. Therefore, a debt collector may demand full payment if you are in default and your contract has an acceleration clause.
Most debt collectors work under private policies and procedures that provide for payment plans. After all, a debt collector would rather get some money every month than no money ever.